Bash Features

This text is a brief description of the features that are present in
the Bash shell (version 5.0, 12 May 2019).
The Bash home page is http://www.gnu.org/software/bash/.

This is Edition 5.0, last updated 12 May 2019,
of The GNU Bash Reference Manual,
for Bash, Version 5.0.

Bash contains features that appear in other popular shells, and some
features that only appear in Bash. Some of the shells that Bash has
borrowed concepts from are the Bourne Shell (sh), the Korn Shell
(ksh), and the C-shell (csh and its successor,
tcsh). The following menu breaks the features up into
categories, noting which features were inspired by other shells and
which are specific to Bash.

This manual is meant as a brief introduction to features found in
Bash. The Bash manual page should be used as the definitive
reference on shell behavior.

1 Introduction

1.1 What is Bash?

Bash is the shell, or command language interpreter,
for the GNU operating system.
The name is an acronym for the ‘Bourne-Again SHell’,
a pun on Stephen Bourne, the author of the direct ancestor of
the current Unix shell sh,
which appeared in the Seventh Edition Bell Labs Research version
of Unix.

Bash is largely compatible with sh and incorporates useful
features from the Korn shell ksh and the C shell csh.
It is intended to be a conformant implementation of the IEEEPOSIX Shell and Tools portion of the IEEEPOSIX
specification (IEEE Standard 1003.1).
It offers functional improvements over sh for both interactive and
programming use.

While the GNU operating system provides other shells, including
a version of csh, Bash is the default shell.
Like other GNU software, Bash is quite portable. It currently runs
on nearly every version of Unix and a few other operating systems -
independently-supported ports exist for MS-DOS, OS/2,
and Windows platforms.

1.2 What is a shell?

At its base, a shell is simply a macro processor that executes
commands. The term macro processor means functionality where text
and symbols are expanded to create larger expressions.

A Unix shell is both a command interpreter and a programming
language. As a command interpreter, the shell provides the user
interface to the rich set of GNU utilities. The programming
language features allow these utilities to be combined.
Files containing commands can be created, and become
commands themselves. These new commands have the same status as
system commands in directories such as /bin, allowing users
or groups to establish custom environments to automate their common
tasks.

Shells may be used interactively or non-interactively. In
interactive mode, they accept input typed from the keyboard.
When executing non-interactively, shells execute commands read
from a file.

A shell allows execution of GNU commands, both synchronously and
asynchronously.
The shell waits for synchronous commands to complete before accepting
more input; asynchronous commands continue to execute in parallel
with the shell while it reads and executes additional commands.
The redirection constructs permit
fine-grained control of the input and output of those commands.
Moreover, the shell allows control over the contents of commands’
environments.

Shells also provide a small set of built-in
commands (builtins) implementing functionality impossible
or inconvenient to obtain via separate utilities.
For example, cd, break, continue, and
exec cannot be implemented outside of the shell because
they directly manipulate the shell itself.
The history, getopts, kill, or pwd
builtins, among others, could be implemented in separate utilities,
but they are more convenient to use as builtin commands.
All of the shell builtins are described in
subsequent sections.

While executing commands is essential, most of the power (and
complexity) of shells is due to their embedded programming
languages. Like any high-level language, the shell provides
variables, flow control constructs, quoting, and functions.

Shells offer features geared specifically for
interactive use rather than to augment the programming language.
These interactive features include job control, command line
editing, command history and aliases. Each of these features is
described in this manual.

3 Basic Shell Features

Bash is an acronym for ‘Bourne-Again SHell’.
The Bourne shell is
the traditional Unix shell originally written by Stephen Bourne.
All of the Bourne shell builtin commands are available in Bash,
The rules for evaluation and quoting are taken from the POSIX
specification for the ‘standard’ Unix shell.

This chapter briefly summarizes the shell’s ‘building blocks’:
commands, control structures, shell functions, shell parameters,
shell expansions,
redirections, which are a way to direct input and output from
and to named files, and how the shell executes commands.

3.1 Shell Syntax

When the shell reads input, it proceeds through a
sequence of operations. If the input indicates the beginning of a
comment, the shell ignores the comment symbol (‘#’), and the rest
of that line.

Otherwise, roughly speaking, the shell reads its input and
divides the input into words and operators, employing the quoting rules
to select which meanings to assign various words and characters.

The shell then parses these tokens into commands and other constructs,
removes the special meaning of certain words or characters, expands
others, redirects input and output as needed, executes the specified
command, waits for the command’s exit status, and makes that exit status
available for further inspection or processing.

Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.

Each of the shell metacharacters (see Definitions)
has special meaning to the shell and must be quoted if it is to
represent itself.
When the command history expansion facilities are being used
(see History Interaction), the
history expansion character, usually ‘!’, must be quoted
to prevent history expansion. See Bash History Facilities, for
more details concerning history expansion.

There are three quoting mechanisms: the
escape character, single quotes, and double quotes.

3.1.2.1 Escape Character

A non-quoted backslash ‘\’ is the Bash escape character.
It preserves the literal value of the next character that follows,
with the exception of newline. If a \newline pair
appears, and the backslash itself is not quoted, the \newline
is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).

3.1.2.3 Double Quotes

Enclosing characters in double quotes (‘"’) preserves the literal value
of all characters within the quotes, with the exception of
‘$’, ‘`’, ‘\’,
and, when history expansion is enabled, ‘!’.
When the shell is in
POSIX mode (see Bash POSIX Mode),
the ‘!’ has no special meaning
within double quotes, even when history expansion is enabled.
The characters ‘$’ and ‘`’
retain their special meaning within double quotes (see Shell Expansions).
The backslash retains its special meaning only when followed by one of
the following characters:
‘$’, ‘`’, ‘"’, ‘\’, or newline.
Within double quotes, backslashes that are followed by one of these
characters are removed. Backslashes preceding characters without a
special meaning are left unmodified.
A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an ‘!’
appearing in double quotes is escaped using a backslash.
The backslash preceding the ‘!’ is not removed.

3.1.2.4 ANSI-C Quoting

Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:

\a

alert (bell)

\b

backspace

\e

\E

an escape character (not ANSI C)

\f

form feed

\n

newline

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\'

single quote

\"

double quote

\?

question mark

\nnn

the eight-bit character whose value is the octal value nnn
(one to three octal digits)

\xHH

the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)

\uHHHH

the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH (one to four hex digits)

3.1.2.5 Locale-Specific Translation

A double-quoted string preceded by a dollar sign (‘$’) will cause
the string to be translated according to the current locale.
If the current locale is C or POSIX, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.

Some systems use the message catalog selected by the LC_MESSAGES
shell variable. Others create the name of the message catalog from the
value of the TEXTDOMAIN shell variable, possibly adding a
suffix of ‘.mo’. If you use the TEXTDOMAIN variable, you
may need to set the TEXTDOMAINDIR variable to the location of
the message catalog files. Still others use both variables in this
fashion:
TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.

3.1.3 Comments

In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt
builtin is enabled (see The Shopt Builtin),
a word beginning with ‘#’
causes that word and all remaining characters on that line to
be ignored. An interactive shell without the interactive_comments
option enabled does not allow comments. The interactive_comments
option is on by default in interactive shells.
See Interactive Shells, for a description of what makes
a shell interactive.

3.2 Shell Commands

A simple shell command such as echo a b c consists of the command
itself followed by arguments, separated by spaces.

More complex shell commands are composed of simple commands arranged together
in a variety of ways: in a pipeline in which the output of one command
becomes the input of a second, in a loop or conditional construct, or in
some other grouping.

3.2.1 Simple Commands

A simple command is the kind of command encountered most often.
It’s just a sequence of words separated by blanks, terminated
by one of the shell’s control operators (see Definitions). The
first word generally specifies a command to be executed, with the
rest of the words being that command’s arguments.

The return status (see Exit Status) of a simple command is
its exit status as provided
by the POSIX 1003.1 waitpid function, or 128+n if
the command was terminated by signal n.

3.2.2 Pipelines

A pipeline is a sequence of one or more commands separated by
one of the control operators ‘|’ or ‘|&’.

The format for a pipeline is

[time [-p]] [!] command1 [ | or |& command2 ] …

The output of each command in the pipeline is connected via a pipe
to the input of the next command.
That is, each command reads the previous command’s output. This
connection is performed before any redirections specified by the
command.

If ‘|&’ is used, command1’s standard error, in addition to
its standard output, is connected to
command2’s standard input through the pipe;
it is shorthand for 2>&1 |.
This implicit redirection of the standard error to the standard output is
performed after any redirections specified by the command.

The reserved word time causes timing statistics
to be printed for the pipeline once it finishes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command’s execution.
The -p option changes the output format to that specified
by POSIX.
When the shell is in POSIX mode (see Bash POSIX Mode),
it does not recognize time as a reserved word if the next
token begins with a ‘-’.
The TIMEFORMAT variable may be set to a format string that
specifies how the timing information should be displayed.
See Bash Variables, for a description of the available formats.
The use of time as a reserved word permits the timing of
shell builtins, shell functions, and pipelines. An external
time command cannot time these easily.

When the shell is in POSIX mode (see Bash POSIX Mode), time
may be followed by a newline. In this case, the shell displays the
total user and system time consumed by the shell and its children.
The TIMEFORMAT variable may be used to specify the format of
the time information.

If the pipeline is not executed asynchronously (see Lists), the
shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell, which is a
separate process (see Command Execution Environment).
If the lastpipe option is enabled using the shopt builtin
(see The Shopt Builtin),
the last element of a pipeline may be run by the shell process.

The exit
status of a pipeline is the exit status of the last command in the
pipeline, unless the pipefail option is enabled
(see The Set Builtin).
If pipefail is enabled, the pipeline’s return status is the
value of the last (rightmost) command to exit with a non-zero status,
or zero if all commands exit successfully.
If the reserved word ‘!’ precedes the pipeline, the
exit status is the logical negation of the exit status as described
above.
The shell waits for all commands in the pipeline to terminate before
returning a value.

3.2.3 Lists of Commands

A list is a sequence of one or more pipelines separated by one
of the operators ‘;’, ‘&’, ‘&&’, or ‘||’,
and optionally terminated by one of ‘;’, ‘&’, or a
newline.

Of these list operators, ‘&&’ and ‘||’
have equal precedence, followed by ‘;’ and ‘&’,
which have equal precedence.

A sequence of one or more newlines may appear in a list
to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator ‘&’,
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the background,
and these are referred to as asynchronous commands.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (see Job Control),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from /dev/null.

Commands separated by a ‘;’ are executed sequentially; the shell
waits for each command to terminate in turn. The return status is the
exit status of the last command executed.

AND and OR lists are sequences of one or more pipelines
separated by the control operators ‘&&’ and ‘||’,
respectively. AND and OR lists are executed with left
associativity.

An AND list has the form

command1 && command2

command2 is executed if, and only if, command1
returns an exit status of zero (success).

An OR list has the form

command1 || command2

command2 is executed if, and only if, command1
returns a non-zero exit status.

The return status of
AND and OR lists is the exit status of the last command
executed in the list.

3.2.4 Compound Commands

Compound commands are the shell programming language constructs.
Each construct begins with a reserved word or control operator and is
terminated by a corresponding reserved word or operator.
Any redirections (see Redirections) associated with a compound command
apply to all commands within that compound command unless explicitly overridden.

In most cases a list of commands in a compound command’s description may be
separated from the rest of the command by one or more newlines, and may be
followed by a newline in place of a semicolon.

Bash provides looping constructs, conditional commands, and mechanisms
to group commands and execute them as a unit.

3.2.4.1 Looping Constructs

Note that wherever a ‘;’ appears in the description of a
command’s syntax, it may be replaced with one or more newlines.

until

The syntax of the until command is:

until test-commands; do consequent-commands; done

Execute consequent-commands as long as
test-commands has an exit status which is not zero.
The return status is the exit status of the last command executed
in consequent-commands, or zero if none was executed.

while

The syntax of the while command is:

while test-commands; do consequent-commands; done

Execute consequent-commands as long as
test-commands has an exit status of zero.
The return status is the exit status of the last command executed
in consequent-commands, or zero if none was executed.

for

The syntax of the for command is:

for name [ [in [words …] ] ; ] do commands; done

Expand words (see Shell Expansions), and execute commands
once for each member
in the resultant list, with name bound to the current member.
If ‘in words’ is not present, the for command
executes the commands once for each positional parameter that is
set, as if ‘in "$@"’ had been specified
(see Special Parameters).

The return status is the exit status of the last command that executes.
If there are no items in the expansion of words, no commands are
executed, and the return status is zero.

An alternate form of the for command is also supported:

for (( expr1 ; expr2 ; expr3 )) ; do commands ; done

First, the arithmetic expression expr1 is evaluated according
to the rules described below (see Shell Arithmetic).
The arithmetic expression expr2 is then evaluated repeatedly
until it evaluates to zero.
Each time expr2 evaluates to a non-zero value, commands are
executed and the arithmetic expression expr3 is evaluated.
If any expression is omitted, it behaves as if it evaluates to 1.
The return value is the exit status of the last command in commands
that is executed, or false if any of the expressions is invalid.

3.2.4.2 Conditional Constructs

The test-commands list is executed, and if its return status is zero,
the consequent-commands list is executed.
If test-commands returns a non-zero status, each elif list
is executed in turn, and if its exit status is zero,
the corresponding more-consequents is executed and the
command completes.
If ‘else alternate-consequents’ is present, and
the final command in the final if or elif clause
has a non-zero exit status, then alternate-consequents is executed.
The return status is the exit status of the last command executed, or
zero if no condition tested true.

case

The syntax of the case command is:

case word in
[ [(] pattern [| pattern]…) command-list ;;]…
esac

case will selectively execute the command-list corresponding to
the first pattern that matches word.
The match is performed according
to the rules described below in Pattern Matching.
If the nocasematch shell option
(see the description of shopt in The Shopt Builtin)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The ‘|’ is used to separate multiple patterns, and the ‘)’
operator terminates a pattern list.
A list of patterns and an associated command-list is known
as a clause.

There may be an arbitrary number of case clauses, each terminated
by a ‘;;’, ‘;&’, or ‘;;&’.
The first pattern that matches determines the
command-list that is executed.
It’s a common idiom to use ‘*’ as the final pattern to define the
default case, since that pattern will always match.

Here is an example using case in a script that could be used to
describe one interesting feature of an animal:

If the ‘;;’ operator is used, no subsequent matches are attempted after
the first pattern match.
Using ‘;&’ in place of ‘;;’ causes execution to continue with
the command-list associated with the next clause, if any.
Using ‘;;&’ in place of ‘;;’ causes the shell to test the patterns
in the next clause, if any, and execute any associated command-list
on a successful match,
continuing the case statement execution as if the pattern list had not matched.

The return status is zero if no pattern is matched. Otherwise, the
return status is the exit status of the command-list executed.

select

The select construct allows the easy generation of menus.
It has almost the same syntax as the for command:

select name [in words …]; do commands; done

The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard
error output stream, each preceded by a number. If the
‘in words’ is omitted, the positional parameters are printed,
as if ‘in "$@"’ had been specified.
The PS3 prompt is then displayed and a line is read from the
standard input.
If the line consists of a number corresponding to one of the displayed
words, then the value of name is set to that word.
If the line is empty, the words and prompt are displayed again.
If EOF is read, the select command completes.
Any other value read causes name to be set to null.
The line read is saved in the variable REPLY.

The commands are executed after each selection until a
break command is executed, at which
point the select command completes.

Here is an example that allows the user to pick a filename from the
current directory, and displays the name and index of the file
selected.

select fname in *;
do
echo you picked $fname \($REPLY\)
break;
done

((…))

(( expression ))

The arithmetic expression is evaluated according to the rules
described below (see Shell Arithmetic).
If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1. This is exactly equivalent to

Return a status of 0 or 1 depending on the evaluation of
the conditional expression expression.
Expressions are composed of the primaries described below in
Bash Conditional Expressions.
Word splitting and filename expansion are not performed on the words
between the [[ and ]]; tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal are performed.
Conditional operators such as ‘-f’ must be unquoted to be recognized
as primaries.

When used with [[, the ‘<’ and ‘>’ operators sort
lexicographically using the current locale.

When the ‘==’ and ‘!=’ operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below in Pattern Matching,
as if the extglob shell option were enabled.
The ‘=’ operator is identical to ‘==’.
If the nocasematch shell option
(see the description of shopt in The Shopt Builtin)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The return value is 0 if the string matches (‘==’) or does not
match (‘!=’) the pattern, and 1 otherwise.
Any part of the pattern may be quoted to force the quoted portion
to be matched as a string.

An additional binary operator, ‘=~’, is available, with the same
precedence as ‘==’ and ‘!=’.
When it is used, the string to the right of the operator is considered
a POSIX extended regular expression and matched accordingly
(as in regex3)).
The return value is 0 if the string matches
the pattern, and 1 otherwise.
If the regular expression is syntactically incorrect, the conditional
expression’s return value is 2.
If the nocasematch shell option
(see the description of shopt in The Shopt Builtin)
is enabled, the match is performed without regard to the case
of alphabetic characters.
Any part of the pattern may be quoted to force the quoted portion
to be matched as a string.
Bracket expressions in regular expressions must be treated carefully,
since normal quoting characters lose their meanings between brackets.
If the pattern is stored in a shell variable, quoting the variable
expansion forces the entire pattern to be matched as a string.
Substrings matched by parenthesized subexpressions within the regular
expression are saved in the array variable BASH_REMATCH.
The element of BASH_REMATCH with index 0 is the portion of the string
matching the entire regular expression.
The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized subexpression.

For example, the following will match a line
(stored in the shell variable line)
if there is a sequence of characters in the value consisting of
any number, including zero, of
space characters, zero or one instances of ‘a’, then a ‘b’:

[[ $line =~ [[:space:]]*?(a)b ]]

That means values like ‘aab’ and ‘ aaaaaab’ will match, as
will a line containing a ‘b’ anywhere in its value.

Storing the regular expression in a shell variable is often a useful
way to avoid problems with quoting characters that are special to the
shell.
It is sometimes difficult to specify a regular expression literally
without using quotes, or to keep track of the quoting used by regular
expressions while paying attention to the shell’s quote removal.
Using a shell variable to store the pattern decreases these problems.
For example, the following is equivalent to the above:

pattern='[[:space:]]*?(a)b'
[[ $line =~ $pattern ]]

If you want to match a character that’s special to the regular expression
grammar, it has to be quoted to remove its special meaning.
This means that in the pattern ‘xxx.txt’, the ‘.’ matches any
character in the string (its usual regular expression meaning), but in the
pattern ‘"xxx.txt"’ it can only match a literal ‘.’.
Shell programmers should take special care with backslashes, since backslashes
are used both by the shell and regular expressions to remove the special
meaning from the following character.
The following two sets of commands are not equivalent:

The first two matches will succeed, but the second two will not, because
in the second two the backslash will be part of the pattern to be matched.
In the first two examples, the backslash removes the special meaning from
‘.’, so the literal ‘.’ matches.
If the string in the first examples were anything other than ‘.’, say
‘a’, the pattern would not match, because the quoted ‘.’ in the
pattern loses its special meaning of matching any single character.

Expressions may be combined using the following operators, listed
in decreasing order of precedence:

( expression )

Returns the value of expression.
This may be used to override the normal precedence of operators.

! expression

True if expression is false.

expression1 && expression2

True if both expression1 and expression2 are true.

expression1 || expression2

True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.

3.2.4.3 Grouping Commands

Bash provides two ways to group a list of commands to be executed
as a unit. When commands are grouped, redirections may be applied
to the entire command list. For example, the output of all the
commands in the list may be redirected to a single stream.

()

( list )

Placing a list of commands between parentheses causes a subshell
environment to be created (see Command Execution Environment), and each
of the commands in list to be executed in that subshell. Since the
list is executed in a subshell, variable assignments do not remain in
effect after the subshell completes.

{}

{ list; }

Placing a list of commands between curly braces causes the list to
be executed in the current shell context. No subshell is created.
The semicolon (or newline) following list is required.

In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons. The braces
are reserved words, so they must be separated from the list
by blanks or other shell metacharacters.
The parentheses are operators, and are
recognized as separate tokens by the shell even if they are not separated
from the list by whitespace.

The exit status of both of these constructs is the exit status of
list.

3.2.5 Coprocesses

A coprocess is a shell command preceded by the coproc
reserved word.
A coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the ‘&’ control operator, with a two-way pipe
established between the executing shell and the coprocess.

The format for a coprocess is:

coproc [NAME] command [redirections]

This creates a coprocess named NAME.
If NAME is not supplied, the default name is COPROC.
NAME must not be supplied if command is a simple
command (see Simple Commands); otherwise, it is interpreted as
the first word of the simple command.

When the coprocess is executed, the shell creates an array variable
(see Arrays)
named NAME in the context of the executing shell.
The standard output of command
is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to NAME[0].
The standard input of command
is connected via a pipe to a file descriptor in the executing shell,
and that file descriptor is assigned to NAME[1].
This pipe is established before any redirections specified by the
command (see Redirections).
The file descriptors can be utilized as arguments to shell commands
and redirections using standard word expansions.
Other than those created to execute command and process substitutions,
the file descriptors are not available in subshells.

The process ID of the shell spawned to execute the coprocess is
available as the value of the variable NAME_PID.
The wait
builtin command may be used to wait for the coprocess to terminate.

Since the coprocess is created as an asynchronous command,
the coproc command always returns success.
The return status of a coprocess is the exit status of command.

3.2.6 GNU Parallel

There are ways to run commands in parallel that are not built into Bash.
GNU Parallel is a tool to do just that.

GNU Parallel, as its name suggests, can be used to build and run commands
in parallel. You may run the same command with different arguments, whether
they are filenames, usernames, hostnames, or lines read from files. GNU
Parallel provides shorthand references to many of the most common operations
(input lines, various portions of the input line, different ways to specify
the input source, and so on). Parallel can replace xargs or feed
commands from its input sources to several different instances of Bash.

For a complete description, refer to the GNU Parallel documentation. A few
examples should provide a brief introduction to its use.

For example, it is easy to replace xargs to gzip all html files in the
current directory and its subdirectories:

find . -type f -name '*.html' -print | parallel gzip

If you need to protect special characters such as newlines in file names,
use find’s -print0 option and parallel’s -0 option.

You can use Parallel to move files from the current directory when the
number of files is too large to process with one mv invocation:

ls | parallel mv {} destdir

As you can see, the {} is replaced with each line read from standard input.
While using ls will work in most instances, it is not sufficient to
deal with all filenames.
If you need to accommodate special characters in filenames, you can use

find . -depth 1 \! -name '.*' -print0 | parallel -0 mv {} destdir

as alluded to above.

This will run as many mv commands as there are files in the current
directory.
You can emulate a parallel xargs by adding the -X option:

find . -depth 1 \! -name '.*' -print0 | parallel -0 -X mv {} destdir

GNU Parallel can replace certain common idioms that operate on lines read
from a file (in this case, filenames listed one per line):

This will recompress all files in the current directory with names ending
in .gz using bzip2, running one job per CPU (-j+0) in parallel.
(We use ls for brevity here; using find as above is more
robust in the face of filenames containing unexpected characters.)
Parallel can take arguments from the command line; the above can also be
written as

parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz

If a command generates output, you may want to preserve the input order in
the output. For instance, the following command

will ensure that the output of traceroute foss.org.my is displayed first.

Finally, Parallel can be used to run a sequence of shell commands in parallel,
similar to ‘cat file | bash’.
It is not uncommon to take a list of filenames, create a series of shell
commands to operate on them, and feed that list of commands to a shell.
Parallel can speed this up. Assuming that file contains a list of
shell commands, one per line,

parallel -j 10 < file

will evaluate the commands using the shell (since no explicit command is
supplied as an argument), in blocks of ten shell jobs at a time.

3.3 Shell Functions

Shell functions are a way to group commands for later execution
using a single name for the group. They are executed just like
a "regular" command.
When the name of a shell function is used as a simple command name,
the list of commands associated with that function name is executed.
Shell functions are executed in the current
shell context; no new process is created to interpret them.

Functions are declared using this syntax:

name () compound-command [ redirections ]

or

function name [()] compound-command [ redirections ]

This defines a shell function named name. The reserved
word function is optional.
If the function reserved
word is supplied, the parentheses are optional.
The body of the function is the compound command
compound-command (see Compound Commands).
That command is usually a list enclosed between { and }, but
may be any compound command listed above,
with one exception: If the function reserved word is used, but the
parentheses are not supplied, the braces are required.
compound-command is executed whenever name is specified as the
name of a command.
When the shell is in POSIX mode (see Bash POSIX Mode),
name may not be the same as one of the special builtins
(see Special Builtins).
Any redirections (see Redirections) associated with the shell function
are performed when the function is executed.

A function definition may be deleted using the -f option to the
unset builtin (see Bourne Shell Builtins).

The exit status of a function definition is zero unless a syntax error
occurs or a readonly function with the same name already exists.
When executed, the exit status of a function is the exit status of the
last command executed in the body.

Note that for historical reasons, in the most common usage the curly braces
that surround the body of the function must be separated from the body by
blanks or newlines.
This is because the braces are reserved words and are only recognized
as such when they are separated from the command list
by whitespace or another shell metacharacter.
Also, when using the braces, the list must be terminated by a semicolon,
a ‘&’, or a newline.

When a function is executed, the arguments to the
function become the positional parameters
during its execution (see Positional Parameters).
The special parameter ‘#’ that expands to the number of
positional parameters is updated to reflect the change.
Special parameter 0 is unchanged.
The first element of the FUNCNAME variable is set to the
name of the function while the function is executing.

All other aspects of the shell execution
environment are identical between a function and its caller
with these exceptions:
the DEBUG and RETURN traps
are not inherited unless the function has been given the
trace attribute using the declare builtin or
the -o functrace option has been enabled with
the set builtin,
(in which case all functions inherit the DEBUG and RETURN traps),
and the ERR trap is not inherited unless the -o errtrace
shell option has been enabled.
See Bourne Shell Builtins, for the description of the
trap builtin.

The FUNCNEST variable, if set to a numeric value greater
than 0, defines a maximum function nesting level. Function
invocations that exceed the limit cause the entire command to
abort.

If the builtin command return
is executed in a function, the function completes and
execution resumes with the next command after the function
call.
Any command associated with the RETURN trap is executed
before execution resumes.
When a function completes, the values of the
positional parameters and the special parameter ‘#’
are restored to the values they had prior to the function’s
execution. If a numeric argument is given to return,
that is the function’s return status; otherwise the function’s
return status is the exit status of the last command executed
before the return.

Variables local to the function may be declared with the
local builtin. These variables are visible only to
the function and the commands it invokes. This is particularly
important when a shell function calls other functions.

Local variables "shadow" variables with the same name declared at
previous scopes. For instance, a local variable declared in a function
hides a global variable of the same name: references and assignments
refer to the local variable, leaving the global variable unmodified.
When the function returns, the global variable is once again visible.

The shell uses dynamic scoping to control a variable’s visibility
within functions.
With dynamic scoping, visible variables and their values
are a result of the sequence of function calls that caused execution
to reach the current function.
The value of a variable that a function sees depends
on its value within its caller, if any, whether that caller is
the "global" scope or another shell function.
This is also the value that a local variable
declaration "shadows", and the value that is restored when the function
returns.

For example, if a variable var is declared as local in function
func1, and func1 calls another function func2,
references to var made from within func2 will resolve to the
local variable var from func1, shadowing any global variable
named var.

The following script demonstrates this behavior.
When executed, the script displays

The unset builtin also acts using the same dynamic scope: if a
variable is local to the current scope, unset will unset it;
otherwise the unset will refer to the variable found in any calling scope
as described above.
If a variable at the current local scope is unset, it will remain so
until it is reset in that scope or until the function returns.
Once the function returns, any instance of the variable at a previous
scope will become visible.
If the unset acts on a variable at a previous scope, any instance of a
variable with that name that had been shadowed will become visible.

Function names and definitions may be listed with the
-f option to the declare (typeset)
builtin command (see Bash Builtins).
The -F option to declare or typeset
will list the function names only
(and optionally the source file and line number, if the extdebug
shell option is enabled).
Functions may be exported so that subshells
automatically have them defined with the
-f option to the export builtin
(see Bourne Shell Builtins).

Functions may be recursive.
The FUNCNEST variable may be used to limit the depth of the
function call stack and restrict the number of function invocations.
By default, no limit is placed on the number of recursive calls.

3.4 Shell Parameters

A parameter is an entity that stores values.
It can be a name, a number, or one of the special characters
listed below.
A variable is a parameter denoted by a name.
A variable has a value and zero or more attributes.
Attributes are assigned using the declare builtin command
(see the description of the declare builtin in Bash Builtins).

A parameter is set if it has been assigned a value. The null string is
a valid value. Once a variable is set, it may be unset only by using
the unset builtin command.

A variable may be assigned to by a statement of the form

name=[value]

If value
is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (detailed below). If the variable has its integer
attribute set, then value
is evaluated as an arithmetic expression even if the $((…))
expansion is not used (see Arithmetic Expansion).
Word splitting is not performed, with the exception
of "$@" as explained below.
Filename expansion is not performed.
Assignment statements may also appear as arguments to the
alias,
declare, typeset, export, readonly,
and local builtin commands (declaration commands).
When in POSIX mode (see Bash POSIX Mode), these builtins may appear
in a command after one or more instances of the command builtin
and retain these assignment statement properties.

In the context where an assignment statement is assigning a value
to a shell variable or array index (see Arrays), the ‘+=’
operator can be used to
append to or add to the variable’s previous value.
This includes arguments to builtin commands such as declare that
accept assignment statements (declaration commands).
When ‘+=’ is applied to a variable for which the integer attribute
has been set, value is evaluated as an arithmetic expression and
added to the variable’s current value, which is also evaluated.
When ‘+=’ is applied to an array variable using compound assignment
(see Arrays), the
variable’s value is not unset (as it is when using ‘=’), and new
values are appended to the array beginning at one greater than the array’s
maximum index (for indexed arrays), or added as additional key-value pairs
in an associative array.
When applied to a string-valued variable, value is expanded and
appended to the variable’s value.

A variable can be assigned the nameref attribute using the
-n option to the declare or local builtin commands
(see Bash Builtins)
to create a nameref, or a reference to another variable.
This allows variables to be manipulated indirectly.
Whenever the nameref variable is referenced, assigned to, unset, or has
its attributes modified (other than using or changing the nameref
attribute itself), the
operation is actually performed on the variable specified by the nameref
variable’s value.
A nameref is commonly used within shell functions to refer to a variable
whose name is passed as an argument to the function.
For instance, if a variable name is passed to a shell function as its first
argument, running

declare -n ref=$1

inside the function creates a nameref variable ref whose value is
the variable name passed as the first argument.
References and assignments to ref, and changes to its attributes,
are treated as references, assignments, and attribute modifications
to the variable whose name was passed as $1.

If the control variable in a for loop has the nameref attribute,
the list of words can be a list of shell variables, and a name reference
will be established for each word in the list, in turn, when the loop is
executed.
Array variables cannot be given the nameref attribute.
However, nameref variables can reference array variables and subscripted
array variables.
Namerefs can be unset using the -n option to the unset builtin
(see Bourne Shell Builtins).
Otherwise, if unset is executed with the name of a nameref variable
as an argument, the variable referenced by the nameref variable will be unset.

3.4.1 Positional Parameters

A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parameters are
assigned from the shell’s arguments when it is invoked,
and may be reassigned using the set builtin command.
Positional parameter N may be referenced as ${N}, or
as $N when N consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The set and shift builtins are used to set and
unset them (see Shell Builtin Commands).
The positional parameters are
temporarily replaced when a shell function is executed
(see Shell Functions).

When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces.

3.4.2 Special Parameters

The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.

*

($*) Expands to the positional parameters, starting from one.
When the expansion is not within double quotes, each positional parameter
expands to a separate word.
In contexts where it is performed, those words
are subject to further word splitting and pathname expansion.
When the expansion occurs within double quotes, it expands to a single word
with the value of each parameter separated by the first character of the
IFS special variable. That is, "$*" is equivalent
to "$1c$2c…", where c
is the first character of the value of the IFS
variable.
If IFS is unset, the parameters are separated by spaces.
If IFS is null, the parameters are joined without intervening
separators.

@

($@) Expands to the positional parameters, starting from one.
In contexts where word splitting is performed, this expands each
positional parameter to a separate word; if not within double
quotes, these words are subject to word splitting.
In contexts where word splitting is not performed,
this expands to a single word
with each positional parameter separated by a space.
When the
expansion occurs within double quotes, and word splitting is performed,
each parameter expands to a
separate word. That is, "$@" is equivalent to
"$1" "$2" ….
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
When there are no positional parameters, "$@" and
$@
expand to nothing (i.e., they are removed).

#

($#) Expands to the number of positional parameters in decimal.

?

($?) Expands to the exit status of the most recently executed foreground
pipeline.

-

($-, a hyphen.) Expands to the current option flags as specified upon
invocation, by the set
builtin command, or those set by the shell itself
(such as the -i option).

$

($$) Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the invoking shell, not the subshell.

!

($!) Expands to the process ID of the job most recently placed into the
background, whether executed as an asynchronous command or using
the bg builtin (see Job Control Builtins).

0

($0) Expands to the name of the shell or shell script. This is set at
shell initialization. If Bash is invoked with a file of commands
(see Shell Scripts), $0 is set to the name of that file.
If Bash is started with the -c option (see Invoking Bash),
then $0 is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.

_

($_, an underscore.)
At shell startup, set to the absolute pathname used to invoke the
shell or shell script being executed as passed in the environment
or argument list.
Subsequently, expands to the last argument to the previous simple
command executed in the foreground, after expansion.
Also set to the full pathname used to invoke each command executed
and placed in the environment exported to that command.
When checking mail, this parameter holds the name of the mail file.

On systems that can support it, there is an additional expansion
available: process substitution.
This is performed at the
same time as tilde, parameter, variable, and arithmetic expansion and
command substitution.

After these expansions are performed, quote characters present in the
original word are removed unless they have been quoted themselves
(quote removal).

Only brace expansion, word splitting, and filename expansion
can increase the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@" and $* (see Special Parameters), and
"${name[@]}" and ${name[*]}
(see Arrays).

3.5.1 Brace Expansion

Brace expansion is a mechanism by which arbitrary strings may be generated.
This mechanism is similar to
filename expansion (see Filename Expansion),
but the filenames generated need not exist.
Patterns to be brace expanded take the form of an optional preamble,
followed by either a series of comma-separated strings or a sequence expression
between a pair of braces,
followed by an optional postscript.
The preamble is prefixed to each string contained within the braces, and
the postscript is then appended to each resulting string, expanding left
to right.

Brace expansions may be nested.
The results of each expanded string are not sorted; left to right order
is preserved.
For example,

bash$ echo a{d,c,b}e
ade ace abe

A sequence expression takes the form {x..y[..incr]},
where x and y are either integers or single characters,
and incr, an optional increment, is an integer.
When integers are supplied, the expression expands to each number between
x and y, inclusive.
Supplied integers may be prefixed with ‘0’ to force each term to have the
same width.
When either x or y begins with a zero, the shell
attempts to force all generated terms to contain the same number of digits,
zero-padding where necessary.
When characters are supplied, the expression expands to each character
lexicographically between x and y, inclusive,
using the default C locale.
Note that both x and y must be of the same type.
When the increment is supplied, it is used as the difference between
each term. The default increment is 1 or -1 as appropriate.

Brace expansion is performed before any other expansions,
and any characters special to other expansions are preserved
in the result. It is strictly textual. Bash
does not apply any syntactic interpretation to the context of the
expansion or the text between the braces.

A correctly-formed brace expansion must contain unquoted opening
and closing braces, and at least one unquoted comma or a valid
sequence expression.
Any incorrectly formed brace expansion is left unchanged.

A { or ‘,’ may be quoted with a backslash to prevent its
being considered part of a brace expression.
To avoid conflicts with parameter expansion, the string ‘${’
is not considered eligible for brace expansion,
and inhibits brace expansion until the closing ‘}’.

This construct is typically used as shorthand when the common
prefix of the strings to be generated is longer than in the
above example:

3.5.2 Tilde Expansion

If a word begins with an unquoted tilde character (‘~’), all of the
characters up to the first unquoted slash (or all characters,
if there is no unquoted slash) are considered a tilde-prefix.
If none of the characters in the tilde-prefix are quoted, the
characters in the tilde-prefix following the tilde are treated as a
possible login name.
If this login name is the null string, the tilde is replaced with the
value of the HOME shell variable.
If HOME is unset, the home directory of the user executing the
shell is substituted instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.

If the tilde-prefix is ‘~+’, the value of
the shell variable PWD replaces the tilde-prefix.
If the tilde-prefix is ‘~-’, the value of the shell variable
OLDPWD, if it is set, is substituted.

If the characters following the tilde in the tilde-prefix consist of a
number N, optionally prefixed by a ‘+’ or a ‘-’,
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the dirs builtin invoked with the characters following tilde
in the tilde-prefix as an argument (see The Directory Stack).
If the tilde-prefix, sans the tilde, consists of a number without a
leading ‘+’ or ‘-’, ‘+’ is assumed.

If the login name is invalid, or the tilde expansion fails, the word is
left unchanged.

Each variable assignment is checked for unquoted tilde-prefixes immediately
following a ‘:’ or the first ‘=’.
In these cases, tilde expansion is also performed.
Consequently, one may use filenames with tildes in assignments to
PATH, MAILPATH, and CDPATH,
and the shell assigns the expanded value.

The following table shows how Bash treats unquoted tilde-prefixes:

~

The value of $HOME

~/foo

$HOME/foo

~fred/foo

The subdirectory foo of the home directory of the user
fred

~+/foo

$PWD/foo

~-/foo

${OLDPWD-'~-'}/foo

~N

The string that would be displayed by ‘dirs +N’

~+N

The string that would be displayed by ‘dirs +N’

~-N

The string that would be displayed by ‘dirs -N’

Bash also performs tilde expansion on words satisfying the conditions of
variable assignments (see Shell Parameters)
when they appear as arguments to simple commands.
Bash does not do this, except for the declaration commands listed
above, when in POSIX mode.

3.5.3 Shell Parameter Expansion

The ‘$’ character introduces parameter expansion,
command substitution, or arithmetic expansion. The parameter name
or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be expanded from
characters immediately following it which could be
interpreted as part of the name.

When braces are used, the matching ending brace is the first ‘}’
not escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.

The basic form of parameter expansion is ${parameter}.
The value of parameter is substituted.
The parameter is a shell parameter as described above
(see Shell Parameters) or an array reference (see Arrays).
The braces are required when parameter
is a positional parameter with more than one digit,
or when parameter is followed by a character that is not to be
interpreted as part of its name.

If the first character of parameter is an exclamation point (!),
and parameter is not a nameref,
it introduces a level of indirection.
Bash uses the value formed by expanding the rest of
parameter as the new parameter; this is then
expanded and that value is used in the rest of the expansion, rather
than the expansion of the original parameter.
This is known as indirect expansion.
The value is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
If parameter is a nameref, this expands to the name of the
variable referenced by parameter instead of performing the
complete indirect expansion.
The exceptions to this are the expansions of ${!prefix*}
and ${!name[@]}
described below.
The exclamation point must immediately follow the left brace in order to
introduce indirection.

In each of the cases below, word is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the form described
below (e.g., ‘:-’), Bash tests for a parameter that is unset or null.
Omitting the colon results in a test only for a parameter that is unset.
Put another way, if the colon is included,
the operator tests for both parameter’s existence and that its value
is not null; if the colon is omitted, the operator tests only for existence.

${parameter:-word}

If parameter is unset or null, the expansion of
word is substituted. Otherwise, the value of
parameter is substituted.

${parameter:=word}

If parameter
is unset or null, the expansion of word
is assigned to parameter.
The value of parameter is then substituted.
Positional parameters and special parameters may not be assigned to
in this way.

${parameter:?word}

If parameter
is null or unset, the expansion of word (or a message
to that effect if word
is not present) is written to the standard error and the shell, if it
is not interactive, exits. Otherwise, the value of parameter is
substituted.

${parameter:+word}

If parameter
is null or unset, nothing is substituted, otherwise the expansion of
word is substituted.

${parameter:offset}

${parameter:offset:length}

This is referred to as Substring Expansion.
It expands to up to length characters of the value of parameter
starting at the character specified by offset.
If parameter is ‘@’, an indexed array subscripted by
‘@’ or ‘*’, or an associative array name, the results differ as
described below.
If length is omitted, it expands to the substring of the value of
parameter starting at the character specified by offset
and extending to the end of the value.
length and offset are arithmetic expressions
(see Shell Arithmetic).

If offset evaluates to a number less than zero, the value
is used as an offset in characters
from the end of the value of parameter.
If length evaluates to a number less than zero,
it is interpreted as an offset in characters
from the end of the value of parameter rather than
a number of characters, and the expansion is the characters between
offset and that result.
Note that a negative offset must be separated from the colon by at least
one space to avoid being confused with the ‘:-’ expansion.

Here are some examples illustrating substring expansion on parameters and
subscripted arrays:

If parameter is ‘@’, the result is length positional
parameters beginning at offset.
A negative offset is taken relative to one greater than the greatest
positional parameter, so an offset of -1 evaluates to the last positional
parameter.
It is an expansion error if length evaluates to a number less than zero.

The following examples illustrate substring expansion using positional
parameters:

If parameter is an indexed array name subscripted
by ‘@’ or ‘*’, the result is the length
members of the array beginning with ${parameter[offset]}.
A negative offset is taken relative to one greater than the maximum
index of the specified array.
It is an expansion error if length evaluates to a number less than zero.

These examples show how you can use substring expansion with indexed
arrays:

Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default.
If offset is 0, and the positional parameters are used, $@ is
prefixed to the list.

${!prefix*}

${!prefix@}

Expands to the names of variables whose names begin with prefix,
separated by the first character of the IFS special variable.
When ‘@’ is used and the expansion appears within double quotes, each
variable name expands to a separate word.

${!name[@]}

${!name[*]}

If name is an array variable, expands to the list of array indices
(keys) assigned in name.
If name is not an array, expands to 0 if name is set and null
otherwise.
When ‘@’ is used and the expansion appears within double quotes, each
key expands to a separate word.

${#parameter}

The length in characters of the expanded value of parameter is
substituted.
If parameter is ‘*’ or ‘@’, the value substituted
is the number of positional parameters.
If parameter is an array name subscripted by ‘*’ or ‘@’,
the value substituted is the number of elements in the array.
If parameter
is an indexed array name subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
parameter, so negative indices count back from the end of the
array, and an index of -1 references the last element.

${parameter#word}

${parameter##word}

The word
is expanded to produce a pattern and matched according to the rules
described below (see Pattern Matching). If the pattern matches
the beginning of the expanded value of parameter,
then the result of the expansion is the expanded value of parameter
with the shortest matching pattern (the ‘#’ case) or the
longest matching pattern (the ‘##’ case) deleted.
If parameter is ‘@’ or ‘*’,
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter is an array variable subscripted with
‘@’ or ‘*’,
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.

${parameter%word}

${parameter%%word}

The word
is expanded to produce a pattern and matched according to the rules
described below (see Pattern Matching).
If the pattern matches a trailing portion of the expanded value of
parameter, then the result of the expansion is the value of
parameter with the shortest matching pattern (the ‘%’ case)
or the longest matching pattern (the ‘%%’ case) deleted.
If parameter is ‘@’ or ‘*’,
the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter
is an array variable subscripted with ‘@’ or ‘*’,
the pattern removal operation is applied to each member of the
array in turn, and the expansion is the resultant list.

${parameter/pattern/string}

The pattern is expanded to produce a pattern just as in
filename expansion.
Parameter is expanded and the longest match of pattern
against its value is replaced with string.
The match is performed according to the rules described below
(see Pattern Matching).
If pattern begins with ‘/’, all matches of pattern are
replaced with string. Normally only the first match is replaced.
If pattern begins with ‘#’, it must match at the beginning
of the expanded value of parameter.
If pattern begins with ‘%’, it must match at the end
of the expanded value of parameter.
If string is null, matches of pattern are deleted
and the / following pattern may be omitted.
If the nocasematch shell option
(see the description of shopt in The Shopt Builtin)
is enabled, the match is performed without regard to the case
of alphabetic characters.
If parameter is ‘@’ or ‘*’,
the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter
is an array variable subscripted with ‘@’ or ‘*’,
the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.

${parameter^pattern}

${parameter^^pattern}

${parameter,pattern}

${parameter,,pattern}

This expansion modifies the case of alphabetic characters in parameter.
The pattern is expanded to produce a pattern just as in
filename expansion.
Each character in the expanded value of parameter is tested against
pattern, and, if it matches the pattern, its case is converted.
The pattern should not attempt to match more than one character.
The ‘^’ operator converts lowercase letters matching pattern
to uppercase; the ‘,’ operator converts matching uppercase letters
to lowercase.
The ‘^^’ and ‘,,’ expansions convert each matched character in the
expanded value; the ‘^’ and ‘,’ expansions match and convert only
the first character in the expanded value.
If pattern is omitted, it is treated like a ‘?’, which matches
every character.
If parameter is ‘@’ or ‘*’,
the case modification operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter
is an array variable subscripted with ‘@’ or ‘*’,
the case modification operation is applied to each member of the
array in turn, and the expansion is the resultant list.

${parameter@operator}

The expansion is either a transformation of the value of parameter
or information about parameter itself, depending on the value of
operator. Each operator is a single letter:

Q

The expansion is a string that is the value of parameter quoted in a
format that can be reused as input.

E

The expansion is a string that is the value of parameter with backslash
escape sequences expanded as with the $'…' quoting mechanism.

P

The expansion is a string that is the result of expanding the value of
parameter as if it were a prompt string (see Controlling the Prompt).

A

The expansion is a string in the form of
an assignment statement or declare command that, if
evaluated, will recreate parameter with its attributes and value.

a

The expansion is a string consisting of flag values representing
parameter’s attributes.

If parameter is ‘@’ or ‘*’,
the operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter
is an array variable subscripted with ‘@’ or ‘*’,
the operation is applied to each member of the
array in turn, and the expansion is the resultant list.

The result of the expansion is subject to word splitting and pathname
expansion as described below.

3.5.4 Command Substitution

Command substitution allows the output of a command to replace
the command itself.
Command substitution occurs when a command is enclosed as follows:

$(command)

or

`command`

Bash performs the expansion by executing command in a subshell environment
and replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution $(cat file) can be
replaced by the equivalent but faster $(< file).

When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
‘$’, ‘`’, or ‘\’.
The first backquote not preceded by a backslash terminates the
command substitution.
When using the $(command) form, all characters between
the parentheses make up the command; none are treated specially.

Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, word splitting and
filename expansion are not performed on the results.

3.5.5 Arithmetic Expansion

Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic expansion is:

$(( expression ))

The expression is treated as if it were within double quotes, but
a double quote inside the parentheses is not treated specially.
All tokens in the expression undergo parameter and variable expansion,
command substitution, and quote removal.
The result is treated as the arithmetic expression to be evaluated.
Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below
(see Shell Arithmetic).
If the expression is invalid, Bash prints a message indicating
failure to the standard error and no substitution occurs.

3.5.6 Process Substitution

Process substitution allows a process’s input or output to be
referred to using a filename.
It takes the form of

<(list)

or

>(list)

The process list is run asynchronously, and its input or output
appears as a filename.
This filename is
passed as an argument to the current command as the result of the
expansion.
If the >(list) form is used, writing to
the file will provide input for list. If the
<(list) form is used, the file passed as an
argument should be read to obtain the output of list.
Note that no space may appear between the < or >
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.
Process substitution is supported on systems that support named
pipes (FIFOs) or the /dev/fd method of naming open files.

When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.

3.5.7 Word Splitting

The shell scans the results of parameter expansion, command substitution,
and arithmetic expansion that did not occur within double quotes for
word splitting.

The shell treats each character of $IFS as a delimiter, and splits
the results of the other expansions into words using these characters
as field terminators.
If IFS is unset, or its value is exactly <space><tab><newline>,
the default, then sequences of
<space>, <tab>, and <newline>
at the beginning and end of the results of the previous
expansions are ignored, and any sequence of IFS
characters not at the beginning or end serves to delimit words.
If IFS has a value other than the default, then sequences of
the whitespace characters space, tab, and newline
are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of IFS (an IFS whitespace character).
Any character in IFS that is not IFS
whitespace, along with any adjacent IFS
whitespace characters, delimits a field. A sequence of IFS
whitespace characters is also treated as a delimiter.
If the value of IFS is null, no word splitting occurs.

Explicit null arguments ("" or '') are retained
and passed to commands as empty strings.
Unquoted implicit null arguments, resulting from the expansion of
parameters that have no values, are removed.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained
and passed to a command as an empty string.
When a quoted null argument appears as part of a word whose expansion is
non-null, the null argument is removed.
That is, the word
-d'' becomes -d after word splitting and
null argument removal.

3.5.8 Filename Expansion

After word splitting, unless the -f option has been set
(see The Set Builtin), Bash scans each word for the characters
‘*’, ‘?’, and ‘[’.
If one of these characters appears, then the word is
regarded as a pattern,
and replaced with an alphabetically sorted list of
filenames matching the pattern (see Pattern Matching).
If no matching filenames are found,
and the shell option nullglob is disabled, the word is left
unchanged.
If the nullglob option is set, and no matches are found, the word
is removed.
If the failglob shell option is set, and no matches are found,
an error message is printed and the command is not executed.
If the shell option nocaseglob is enabled, the match is performed
without regard to the case of alphabetic characters.

When a pattern is used for filename expansion, the character ‘.’
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option dotglob is set.
The filenames ‘.’ and ‘..’ must always be matched explicitly,
even if dotglob is set.
In other cases, the ‘.’ character is not treated specially.

When matching a filename, the slash character must always be
matched explicitly by a slash in the pattern, but in other matching
contexts it can be matched by a special pattern character as described
below (see Pattern Matching).

See the description of shopt in The Shopt Builtin,
for a description of the nocaseglob, nullglob,
failglob, and dotglob options.

The GLOBIGNORE
shell variable may be used to restrict the set of file names matching a
pattern. If GLOBIGNORE
is set, each matching file name that also matches one of the patterns in
GLOBIGNORE is removed from the list of matches.
If the nocaseglob option is set, the matching against the patterns in
GLOBIGNORE is performed without regard to case.
The filenames
. and ..
are always ignored when GLOBIGNORE
is set and not null.
However, setting GLOBIGNORE to a non-null value has the effect of
enabling the dotglob
shell option, so all other filenames beginning with a
‘.’ will match.
To get the old behavior of ignoring filenames beginning with a
‘.’, make ‘.*’ one of the patterns in GLOBIGNORE.
The dotglob option is disabled when GLOBIGNORE
is unset.

3.5.8.1 Pattern Matching

Any character that appears in a pattern, other than the special pattern
characters described below, matches itself.
The NUL character may not occur in a pattern.
A backslash escapes the following character; the
escaping backslash is discarded when matching.
The special pattern characters must be quoted if they are to be matched
literally.

The special pattern characters have the following meanings:

*

Matches any string, including the null string.
When the globstar shell option is enabled, and ‘*’ is used in
a filename expansion context, two adjacent ‘*’s used as a single
pattern will match all files and zero or more directories and
subdirectories.
If followed by a ‘/’, two adjacent ‘*’s will match only
directories and subdirectories.

?

Matches any single character.

[…]

Matches any one of the enclosed characters. A pair of characters
separated by a hyphen denotes a range expression;
any character that falls between those two characters, inclusive,
using the current locale’s collating sequence and character set,
is matched. If the first character following the
‘[’ is a ‘!’ or a ‘^’
then any character not enclosed is matched. A ‘-’
may be matched by including it as the first or last character
in the set. A ‘]’ may be matched by including it as the first
character in the set.
The sorting order of characters in range expressions is determined by
the current locale and the values of the
LC_COLLATE and LC_ALL shell variables, if set.

For example, in the default C locale, ‘[a-dx-z]’ is equivalent to
‘[abcdxyz]’. Many locales sort characters in dictionary order, and in
these locales ‘[a-dx-z]’ is typically not equivalent to ‘[abcdxyz]’;
it might be equivalent to ‘[aBbCcDdxXyYz]’, for example. To obtain
the traditional interpretation of ranges in bracket expressions, you can
force the use of the C locale by setting the LC_COLLATE or
LC_ALL environment variable to the value ‘C’, or enable the
globasciiranges shell option.

Within ‘[’ and ‘]’, character classes can be specified
using the syntax
[:class:], where class is one of the
following classes defined in the POSIX standard:

A character class matches any character belonging to that class.
The word character class matches letters, digits, and the character
‘_’.

Within ‘[’ and ‘]’, an equivalence class can be
specified using the syntax [=c=], which
matches all characters with the same collation weight (as defined
by the current locale) as the character c.

Within ‘[’ and ‘]’, the syntax [.symbol.]
matches the collating symbol symbol.

If the extglob shell option is enabled using the shopt
builtin, several extended pattern matching operators are recognized.
In the following description, a pattern-list is a list of one
or more patterns separated by a ‘|’.
Composite patterns may be formed using one or more of the following
sub-patterns:

?(pattern-list)

Matches zero or one occurrence of the given patterns.

*(pattern-list)

Matches zero or more occurrences of the given patterns.

+(pattern-list)

Matches one or more occurrences of the given patterns.

@(pattern-list)

Matches one of the given patterns.

!(pattern-list)

Matches anything except one of the given patterns.

Complicated extended pattern matching against long strings is slow,
especially when the patterns contain alternations and the strings
contain multiple matches.
Using separate matches against shorter strings, or using arrays of
strings instead of a single long string, may be faster.

3.6 Redirections

Before a command is executed, its input and output
may be redirected
using a special notation interpreted by the shell.
Redirection allows commands’ file handles to be
duplicated, opened, closed,
made to refer to different files,
and can change the files the command reads from and writes to.
Redirection may also be used to modify file handles in the
current shell execution environment. The following redirection
operators may precede or appear anywhere within a
simple command or may follow a command.
Redirections are processed in the order they appear, from
left to right.

Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {varname}.
In this case, for each redirection operator except
>&- and <&-, the shell will allocate a file descriptor greater
than 10 and assign it to {varname}. If >&- or <&- is preceded
by {varname}, the value of varname defines the file
descriptor to close.
If {varname} is supplied, the redirection persists beyond
the scope of the command, allowing the shell programmer to manage
the file descriptor himself.

In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is
‘<’, the redirection refers to the standard input (file
descriptor 0). If the first character of the redirection operator
is ‘>’, the redirection refers to the standard output (file
descriptor 1).

The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter expansion, command substitution, arithmetic
expansion, quote removal, filename expansion, and word splitting.
If it expands to more than one word, Bash reports an error.

Note that the order of redirections is significant. For example,
the command

ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error
(file descriptor 2) to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist,
because the standard error was made a copy of the standard output
before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in
redirections, as described in the following table.
If the operating system on which Bash is running provides these
special files, bash will use them; otherwise it will emulate them
internally with the behavior described below.

/dev/fd/fd

If fd is a valid integer, file descriptor fd is duplicated.

/dev/stdin

File descriptor 0 is duplicated.

/dev/stdout

File descriptor 1 is duplicated.

/dev/stderr

File descriptor 2 is duplicated.

/dev/tcp/host/port

If host is a valid hostname or Internet address, and port
is an integer port number or service name, Bash attempts to open
the corresponding TCP socket.

/dev/udp/host/port

If host is a valid hostname or Internet address, and port
is an integer port number or service name, Bash attempts to open
the corresponding UDP socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with
care, as they may conflict with file descriptors the shell uses
internally.

3.6.1 Redirecting Input

Redirection of input causes the file whose name results from
the expansion of word
to be opened for reading on file descriptor n,
or the standard input (file descriptor 0) if n
is not specified.

The general format for redirecting input is:

[n]<word

3.6.2 Redirecting Output

Redirection of output causes the file whose name results from
the expansion of word
to be opened for writing on file descriptor n,
or the standard output (file descriptor 1) if n
is not specified. If the file does not exist it is created;
if it does exist it is truncated to zero size.

The general format for redirecting output is:

[n]>[|]word

If the redirection operator is ‘>’, and the noclobber
option to the set builtin has been enabled, the redirection
will fail if the file whose name results from the expansion of
word exists and is a regular file.
If the redirection operator is ‘>|’, or the redirection operator is
‘>’ and the noclobber option is not enabled, the redirection
is attempted even if the file named by word exists.

3.6.3 Appending Redirected Output

Redirection of output in this fashion
causes the file whose name results from
the expansion of word
to be opened for appending on file descriptor n,
or the standard output (file descriptor 1) if n
is not specified. If the file does not exist it is created.

The general format for appending output is:

[n]>>word

3.6.4 Redirecting Standard Output and Standard Error

This construct allows both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to be redirected to the file whose name is the
expansion of word.

There are two formats for redirecting standard output and
standard error:

&>word

and

>&word

Of the two forms, the first is preferred.
This is semantically equivalent to

>word 2>&1

When using the second form, word may not expand to a number or
‘-’. If it does, other redirection operators apply
(see Duplicating File Descriptors below) for compatibility reasons.

3.6.5 Appending Standard Output and Standard Error

This construct allows both the
standard output (file descriptor 1) and
the standard error output (file descriptor 2)
to be appended to the file whose name is the
expansion of word.

The format for appending standard output and standard error is:

&>>word

This is semantically equivalent to

>>word 2>&1

(see Duplicating File Descriptors below).

3.6.6 Here Documents

This type of redirection instructs the shell to read input from the
current source until a line containing only word
(with no trailing blanks) is seen. All of
the lines read up to that point are then used as the standard
input (or file descriptor n if n is specified) for a command.

The format of here-documents is:

[n]<<[-]wordhere-documentdelimiter

No parameter and variable expansion, command substitution,
arithmetic expansion, or filename expansion is performed on
word. If any part of word is quoted, the
delimiter is the result of quote removal on word,
and the lines in the here-document are not expanded.
If word is unquoted,
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence \newline is ignored, and ‘\’
must be used to quote the characters
‘\’, ‘$’, and ‘`’.

If the redirection operator is ‘<<-’,
then all leading tab characters are stripped from input lines and the
line containing delimiter.
This allows here-documents within shell scripts to be indented in a
natural fashion.

3.6.7 Here Strings

A variant of here documents, the format is:

[n]<<< word

The word undergoes
tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal.
Pathname expansion and word splitting are not performed.
The result is supplied as a single string,
with a newline appended,
to the command on its
standard input (or file descriptor n if n is specified).

3.6.8 Duplicating File Descriptors

The redirection operator

[n]<&word

is used to duplicate input file descriptors.
If word
expands to one or more digits, the file descriptor denoted by n
is made to be a copy of that file descriptor.
If the digits in word do not specify a file descriptor open for
input, a redirection error occurs.
If word
evaluates to ‘-’, file descriptor n is closed.
If n is not specified, the standard input (file descriptor 0) is used.

The operator

[n]>&word

is used similarly to duplicate output file descriptors. If
n is not specified, the standard output (file descriptor 1) is used.
If the digits in word do not specify a file descriptor open for
output, a redirection error occurs.
If word
evaluates to ‘-’, file descriptor n is closed.
As a special case, if n is omitted, and word does not
expand to one or more digits or ‘-’, the standard output and standard
error are redirected as described previously.

3.6.9 Moving File Descriptors

The redirection operator

[n]<&digit-

moves the file descriptor digit to file descriptor n,
or the standard input (file descriptor 0) if n is not specified.
digit is closed after being duplicated to n.

Similarly, the redirection operator

[n]>&digit-

moves the file descriptor digit to file descriptor n,
or the standard output (file descriptor 1) if n is not specified.

3.6.10 Opening File Descriptors for Reading and Writing

The redirection operator

[n]<>word

causes the file whose name is the expansion of word
to be opened for both reading and writing on file descriptor
n, or on file descriptor 0 if n
is not specified. If the file does not exist, it is created.

3.7.1 Simple Command Expansion

When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.

The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.

The words that are not variable assignments or redirections are
expanded (see Shell Expansions).
If any words remain after expansion, the first word
is taken to be the name of the command and the remaining words are
the arguments.

The text after the ‘=’ in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic expansion,
and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the current
shell environment. Otherwise, the variables are added to the environment
of the executed command and do not affect the current shell environment.
If any of the assignments attempts to assign a value to a readonly variable,
an error occurs, and the command exits with a non-zero status.

If no command name results, redirections are performed, but do not
affect the current shell environment. A redirection error causes the
command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as
described below. Otherwise, the command exits. If one of the expansions
contained a command substitution, the exit status of the command is
the exit status of the last command substitution performed. If there
were no command substitutions, the command exits with a status of zero.

3.7.2 Command Search and Execution

After a command has been split into words, if it results in a
simple command and an optional list of arguments, the following
actions are taken.

If the command name contains no slashes, the shell attempts to
locate it. If there exists a shell function by that name, that
function is invoked as described in Shell Functions.

If the name does not match a function, the shell searches for
it in the list of shell builtins. If a match is found, that
builtin is invoked.

If the name is neither a shell function nor a builtin,
and contains no slashes, Bash searches each element of
$PATH for a directory containing an executable file
by that name. Bash uses a hash table to remember the full
pathnames of executable files to avoid multiple PATH searches
(see the description of hash in Bourne Shell Builtins).
A full search of the directories in $PATH
is performed only if the command is not found in the hash table.
If the search is unsuccessful, the shell searches for a defined shell
function named command_not_found_handle.
If that function exists, it is invoked in a separate execution environment
with the original command and
the original command’s arguments as its arguments, and the function’s
exit status becomes the exit status of that subshell.
If that function is not defined, the shell prints an error
message and returns an exit status of 127.

If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program in
a separate execution environment.
Argument 0 is set to the name given, and the remaining arguments
to the command are set to the arguments supplied, if any.

If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
shell script and the shell executes it as described in
Shell Scripts.

If the command was not begun asynchronously, the shell waits for
the command to complete and collects its exit status.

various process IDs, including those of background jobs
(see Lists), the value of $$, and the value of
$PPID

When a simple command other than a builtin or shell function
is to be executed, it
is invoked in a separate execution environment that consists of
the following. Unless otherwise noted, the values are inherited
from the shell.

the shell’s open files, plus any modifications and additions specified
by redirections to the command

the current working directory

the file creation mode mask

shell variables and functions marked for export, along with variables
exported for the command, passed in the environment (see Environment)

traps caught by the shell are reset to the values inherited from the
shell’s parent, and traps ignored by the shell are ignored

A command invoked in this separate environment cannot affect the
shell’s execution environment.

Command substitution, commands grouped with parentheses,
and asynchronous commands are invoked in a
subshell environment that is a duplicate of the shell environment,
except that traps caught by the shell are reset to the values
that the shell inherited from its parent at invocation. Builtin
commands that are invoked as part of a pipeline are also executed
in a subshell environment. Changes made to the subshell environment
cannot affect the shell’s execution environment.

Subshells spawned to execute command substitutions inherit the value of
the -e option from the parent shell. When not in POSIX mode,
Bash clears the -e option in such subshells.

If a command is followed by a ‘&’ and job control is not active, the
default standard input for the command is the empty file /dev/null.
Otherwise, the invoked command inherits the file descriptors of the calling
shell as modified by redirections.

3.7.4 Environment

When a program is invoked it is given an array of strings
called the environment.
This is a list of name-value pairs, of the form name=value.

Bash provides several ways to manipulate the environment.
On invocation, the shell scans its own environment and
creates a parameter for each name found, automatically marking
it for export
to child processes. Executed commands inherit the environment.
The export and ‘declare -x’
commands allow parameters and functions to be added to and
deleted from the environment. If the value of a parameter
in the environment is modified, the new value becomes part
of the environment, replacing the old. The environment
inherited by any executed command consists of the shell’s
initial environment, whose values may be modified in the shell,
less any pairs removed by the unset and ‘export -n’
commands, plus any additions via the export and
‘declare -x’ commands.

The environment for any simple command
or function may be augmented temporarily by prefixing it with
parameter assignments, as described in Shell Parameters.
These assignment statements affect only the environment seen
by that command.

If the -k option is set (see The Set Builtin), then all
parameter assignments are placed in the environment for a command,
not just those that precede the command name.

When Bash invokes an external command, the variable ‘$_’
is set to the full pathname of the command and passed to that
command in its environment.

3.7.5 Exit Status

The exit status of an executed command is the value returned by the
waitpid system call or equivalent function. Exit statuses
fall between 0 and 255, though, as explained below, the shell may
use values above 125 specially. Exit statuses from shell builtins and
compound commands are also limited to this range. Under certain
circumstances, the shell will use special values to indicate specific
failure modes.

For the shell’s purposes, a command which exits with a
zero exit status has succeeded.
A non-zero exit status indicates failure.
This seemingly counter-intuitive scheme is used so there
is one well-defined way to indicate success and a variety of
ways to indicate various failure modes.
When a command terminates on a fatal signal whose number is N,
Bash uses the value 128+N as the exit status.

If a command is not found, the child process created to
execute it returns a status of 127. If a command is found
but is not executable, the return status is 126.

If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.

All of the Bash builtins return an exit status of zero if they succeed
and a non-zero status on failure, so they may be used by the
conditional and list constructs.
All builtins return an exit status of 2 to indicate incorrect usage,
generally invalid options or missing arguments.

3.7.6 Signals

When Bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that ‘kill 0’ does not kill an interactive shell),
and SIGINT
is caught and handled (so that the wait builtin is interruptible).
When Bash receives a SIGINT, it breaks out of any executing loops.
In all cases, Bash ignores SIGQUIT.
If job control is in effect (see Job Control), Bash
ignores SIGTTIN, SIGTTOU, and SIGTSTP.

Non-builtin commands started by Bash have signal handlers set to the
values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore SIGINT and SIGQUIT in addition to these inherited
handlers.
Commands run as a result of
command substitution ignore the keyboard-generated job control signals
SIGTTIN, SIGTTOU, and SIGTSTP.

The shell exits by default upon receipt of a SIGHUP.
Before exiting, an interactive shell resends the SIGHUP to
all jobs, running or stopped.
Stopped jobs are sent SIGCONT to ensure that they receive
the SIGHUP.
To prevent the shell from sending the SIGHUP signal to a
particular job, it should be removed
from the jobs table with the disown
builtin (see Job Control Builtins) or marked
to not receive SIGHUP using disown -h.

If the huponexit shell option has been set with shopt
(see The Shopt Builtin), Bash sends a SIGHUP to all jobs when
an interactive login shell exits.

If Bash is waiting for a command to complete and receives a signal
for which a trap has been set, the trap will not be executed until
the command completes.
When Bash is waiting for an asynchronous
command via the wait builtin, the reception of a signal for
which a trap has been set will cause the wait builtin to return
immediately with an exit status greater than 128, immediately after
which the trap is executed.

3.8 Shell Scripts

A shell script is a text file containing shell commands. When such
a file is used as the first non-option argument when invoking Bash,
and neither the -c nor -s option is supplied
(see Invoking Bash),
Bash reads and executes commands from the file, then exits. This
mode of operation creates a non-interactive shell. The shell first
searches for the file in the current directory, and looks in the
directories in $PATH if not found there.

When Bash runs
a shell script, it sets the special parameter 0 to the name
of the file, rather than the name of the shell, and the positional
parameters are set to the remaining arguments, if any are given.
If no additional arguments are supplied, the positional parameters
are unset.

A shell script may be made executable by using the chmod command
to turn on the execute bit. When Bash finds such a file while
searching the $PATH for a command, it spawns a subshell to
execute it. In other words, executing

filename arguments

is equivalent to executing

bash filename arguments

if filename is an executable shell script.
This subshell reinitializes itself, so that the effect is as if a
new shell had been invoked to interpret the script, with the
exception that the locations of commands remembered by the parent
(see the description of hash in Bourne Shell Builtins)
are retained by the child.

Most versions of Unix make this a part of the operating system’s command
execution mechanism. If the first line of a script begins with
the two characters ‘#!’, the remainder of the line specifies
an interpreter for the program.
Thus, you can specify Bash, awk, Perl, or some other
interpreter and write the rest of the script file in that language.

The arguments to the interpreter
consist of a single optional argument following the interpreter
name on the first line of the script file, followed by the name of
the script file, followed by the rest of the arguments. Bash
will perform this action on operating systems that do not handle it
themselves. Note that some older versions of Unix limit the interpreter
name and argument to a maximum of 32 characters.

Bash scripts often begin with #! /bin/bash (assuming that
Bash has been installed in /bin), since this ensures that
Bash will be used to interpret the script, even if it is executed
under another shell.

Builtin commands are contained within the shell itself.
When the name of a builtin command is used as the first word of
a simple command (see Simple Commands), the shell executes
the command directly, without invoking another program.
Builtin commands are necessary to implement functionality impossible
or inconvenient to obtain with separate utilities.

This section briefly describes the builtins which Bash inherits from
the Bourne Shell, as well as the builtin commands which are unique
to or have been extended in Bash.

Unless otherwise noted, each builtin command documented as accepting
options preceded by ‘-’ accepts ‘--’
to signify the end of the options.
The :, true, false, and test/[
builtins do not accept options and do not treat ‘--’ specially.
The exit, logout, return,
break, continue, let,
and shift builtins accept and process arguments beginning
with ‘-’ without requiring ‘--’.
Other builtins that accept arguments but are not specified as accepting
options interpret arguments beginning with ‘-’ as invalid options and
require ‘--’ to prevent this interpretation.

4.1 Bourne Shell Builtins

The following shell builtin commands are inherited from the Bourne Shell.
These commands are implemented as specified by the POSIX standard.

: (a colon)

: [arguments]

Do nothing beyond expanding arguments and performing redirections.
The return status is zero.

. (a period)

. filename [arguments]

Read and execute commands from the filename argument in the
current shell context. If filename does not contain a slash,
the PATH variable is used to find filename.
When Bash is not in POSIX mode, the current directory is searched
if filename is not found in $PATH.
If any arguments are supplied, they become the positional
parameters when filename is executed. Otherwise the positional
parameters are unchanged.
If the -T option is enabled, source inherits any trap on
DEBUG; if it is not, any DEBUG trap string is saved and
restored around the call to source, and source unsets the
DEBUG trap while it executes.
If -T is not set, and the sourced file changes
the DEBUG trap, the new value is retained when source completes.
The return status is the exit status of the last command executed, or
zero if no commands are executed. If filename is not found, or
cannot be read, the return status is non-zero.
This builtin is equivalent to source.

break

break [n]

Exit from a for, while, until, or select loop.
If n is supplied, the nth enclosing loop is exited.
n must be greater than or equal to 1.
The return status is zero unless n is not greater than or equal to 1.

cd

cd [-L|[-P [-e]] [-@] [directory]

Change the current working directory to directory.
If directory is not supplied, the value of the HOME
shell variable is used.
Any additional arguments following directory are ignored.
If the shell variable
CDPATH exists, it is used as a search path:
each directory name in CDPATH is searched for
directory, with alternative directory names in CDPATH
separated by a colon (‘:’).
If directory begins with a slash, CDPATH is not used.

The -P option means to not follow symbolic links: symbolic links
are resolved while cd is traversing directory and before
processing an instance of ‘..’ in directory.

By default, or when the -L option is supplied, symbolic links
in directory are resolved after cd processes an instance
of ‘..’ in directory.

If ‘..’ appears in directory, it is processed by removing the
immediately preceding pathname component, back to a slash or the beginning
of directory.

If the -e option is supplied with -P
and the current working directory cannot be successfully determined
after a successful directory change, cd will return an unsuccessful
status.

On systems that support it, the -@ option presents the extended
attributes associated with a file as a directory.

If directory is ‘-’, it is converted to $OLDPWD
before the directory change is attempted.

If a non-empty directory name from CDPATH is used, or if
‘-’ is the first argument, and the directory change is
successful, the absolute pathname of the new working directory is
written to the standard output.

The return status is zero if the directory is successfully changed,
non-zero otherwise.

continue

continue [n]

Resume the next iteration of an enclosing for, while,
until, or select loop.
If n is supplied, the execution of the nth enclosing loop
is resumed.
n must be greater than or equal to 1.
The return status is zero unless n is not greater than or equal to 1.

eval

eval [arguments]

The arguments are concatenated together into a single command, which is
then read and executed, and its exit status returned as the exit status
of eval.
If there are no arguments or only empty arguments, the return status is
zero.

exec

exec [-cl] [-a name] [command [arguments]]

If command
is supplied, it replaces the shell without creating a new process.
If the -l option is supplied, the shell places a dash at the
beginning of the zeroth argument passed to command.
This is what the login program does.
The -c option causes command to be executed with an empty
environment.
If -a is supplied, the shell passes name as the zeroth
argument to command.
If command
cannot be executed for some reason, a non-interactive shell exits,
unless the execfail shell option
is enabled. In that case, it returns failure.
An interactive shell returns failure if the file cannot be executed.
A subshell exits unconditionally if exec fails.
If no command is specified, redirections may be used to affect
the current shell environment. If there are no redirection errors, the
return status is zero; otherwise the return status is non-zero.

exit

exit [n]

Exit the shell, returning a status of n to the shell’s parent.
If n is omitted, the exit status is that of the last command executed.
Any trap on EXIT is executed before the shell terminates.

export

export [-fn] [-p] [name[=value]]

Mark each name to be passed to child processes
in the environment. If the -f option is supplied, the names
refer to shell functions; otherwise the names refer to shell variables.
The -n option means to no longer mark each name for export.
If no names are supplied, or if the -p option is given, a
list of names of all exported variables is displayed.
The -p option displays output in a form that may be reused as input.
If a variable name is followed by =value, the value of
the variable is set to value.

The return status is zero unless an invalid option is supplied, one of
the names is not a valid shell variable name, or -f is supplied
with a name that is not a shell function.

getopts

getopts optstringname [args]

getopts is used by shell scripts to parse positional parameters.
optstring contains the option characters to be recognized; if a
character is followed by a colon, the option is expected to have an
argument, which should be separated from it by whitespace.
The colon (‘:’) and question mark (‘?’) may not be
used as option characters.
Each time it is invoked, getopts
places the next option in the shell variable name, initializing
name if it does not exist,
and the index of the next argument to be processed into the
variable OPTIND.
OPTIND is initialized to 1 each time the shell or a shell script
is invoked.
When an option requires an argument,
getopts places that argument into the variable OPTARG.
The shell does not reset OPTIND automatically; it must be manually
reset between multiple calls to getopts within the same shell
invocation if a new set of parameters is to be used.

When the end of options is encountered, getopts exits with a
return value greater than zero.
OPTIND is set to the index of the first non-option argument,
and name is set to ‘?’.

getopts
normally parses the positional parameters, but if more arguments are
given in args, getopts parses those instead.

getopts can report errors in two ways. If the first character of
optstring is a colon, silent
error reporting is used. In normal operation, diagnostic messages
are printed when invalid options or missing option arguments are
encountered.
If the variable OPTERR
is set to 0, no error messages will be displayed, even if the first
character of optstring is not a colon.

If an invalid option is seen,
getopts places ‘?’ into name and, if not silent,
prints an error message and unsets OPTARG.
If getopts is silent, the option character found is placed in
OPTARG and no diagnostic message is printed.

If a required argument is not found, and getopts
is not silent, a question mark (‘?’) is placed in name,
OPTARG is unset, and a diagnostic message is printed.
If getopts is silent, then a colon (‘:’) is placed in
name and OPTARG is set to the option character found.

hash

hash [-r] [-p filename] [-dt] [name]

Each time hash is invoked, it remembers the full pathnames of the
commands specified as name arguments,
so they need not be searched for on subsequent invocations.
The commands are found by searching through the directories listed in
$PATH.
Any previously-remembered pathname is discarded.
The -p option inhibits the path search, and filename is
used as the location of name.
The -r option causes the shell to forget all remembered locations.
The -d option causes the shell to forget the remembered location
of each name.
If the -t option is supplied, the full pathname to which each
name corresponds is printed. If multiple name arguments are
supplied with -t, the name is printed before the hashed
full pathname.
The -l option causes output to be displayed in a format
that may be reused as input.
If no arguments are given, or if only -l is supplied,
information about remembered commands is printed.
The return status is zero unless a name is not found or an invalid
option is supplied.

pwd

pwd [-LP]

Print the absolute pathname of the current working directory.
If the -P option is supplied, the pathname printed will not
contain symbolic links.
If the -L option is supplied, the pathname printed may contain
symbolic links.
The return status is zero unless an error is encountered while
determining the name of the current directory or an invalid option
is supplied.

readonly

readonly [-aAf] [-p] [name[=value]] …

Mark each name as readonly.
The values of these names may not be changed by subsequent assignment.
If the -f option is supplied, each name refers to a shell
function.
The -a option means each name refers to an indexed
array variable; the -A option means each name refers
to an associative array variable.
If both options are supplied, -A takes precedence.
If no name arguments are given, or if the -p
option is supplied, a list of all readonly names is printed.
The other options may be used to restrict the output to a subset of
the set of readonly names.
The -p option causes output to be displayed in a format that
may be reused as input.
If a variable name is followed by =value, the value of
the variable is set to value.
The return status is zero unless an invalid option is supplied, one of
the name arguments is not a valid shell variable or function name,
or the -f option is supplied with a name that is not a shell function.

return

return [n]

Cause a shell function to stop executing and return the value n
to its caller.
If n is not supplied, the return value is the exit status of the
last command executed in the function.
If return is executed by a trap handler, the last command used to
determine the status is the last command executed before the trap handler.
If return is executed during a DEBUG trap, the last command
used to determine the status is the last command executed by the trap
handler before return was invoked.
return may also be used to terminate execution of a script
being executed with the . (source) builtin,
returning either n or
the exit status of the last command executed within the script as the exit
status of the script.
If n is supplied, the return value is its least significant
8 bits.
Any command associated with the RETURN trap is executed
before execution resumes after the function or script.
The return status is non-zero if return is supplied a non-numeric
argument or is used outside a function
and not during the execution of a script by . or source.

shift

shift [n]

Shift the positional parameters to the left by n.
The positional parameters from n+1 … $# are
renamed to $1 … $#-n.
Parameters represented by the numbers $# to $#-n+1
are unset.
n must be a non-negative number less than or equal to $#.
If n is zero or greater than $#, the positional parameters
are not changed.
If n is not supplied, it is assumed to be 1.
The return status is zero unless n is greater than $# or
less than zero, non-zero otherwise.

test

[

test expr

Evaluate a conditional expression expr and return a status of 0
(true) or 1 (false).
Each operator and operand must be a separate argument.
Expressions are composed of the primaries described below in
Bash Conditional Expressions.
test does not accept any options, nor does it accept and ignore
an argument of -- as signifying the end of options.

When the [ form is used, the last argument to the command must
be a ].

Expressions may be combined using the following operators, listed in
decreasing order of precedence.
The evaluation depends on the number of arguments; see below.
Operator precedence is used when there are five or more arguments.

! expr

True if expr is false.

( expr )

Returns the value of expr.
This may be used to override the normal precedence of operators.

expr1 -a expr2

True if both expr1 and expr2 are true.

expr1 -o expr2

True if either expr1 or expr2 is true.

The test and [ builtins evaluate conditional
expressions using a set of rules based on the number of arguments.

0 arguments

The expression is false.

1 argument

The expression is true if, and only if, the argument is not null.

2 arguments

If the first argument is ‘!’, the expression is true if and
only if the second argument is null.
If the first argument is one of the unary conditional operators
(see Bash Conditional Expressions), the expression
is true if the unary test is true.
If the first argument is not a valid unary operator, the expression is
false.

3 arguments

The following conditions are applied in the order listed.

If the second argument is one of the binary conditional
operators (see Bash Conditional Expressions), the
result of the expression is the result of the binary test using the
first and third arguments as operands.
The ‘-a’ and ‘-o’ operators are considered binary operators
when there are three arguments.

If the first argument is ‘!’, the value is the negation of
the two-argument test using the second and third arguments.

If the first argument is exactly ‘(’ and the third argument is
exactly ‘)’, the result is the one-argument test of the second
argument.

Otherwise, the expression is false.

4 arguments

If the first argument is ‘!’, the result is the negation of
the three-argument expression composed of the remaining arguments.
Otherwise, the expression is parsed and evaluated according to
precedence using the rules listed above.

5 or more arguments

The expression is parsed and evaluated according to precedence
using the rules listed above.

When used with test or ‘[’, the ‘<’ and ‘>’
operators sort lexicographically using ASCII ordering.

times

times

Print out the user and system times used by the shell and its children.
The return status is zero.

trap

trap [-lp] [arg] [sigspec …]

The commands in arg are to be read and executed when the
shell receives signal sigspec. If arg is absent (and
there is a single sigspec) or
equal to ‘-’, each specified signal’s disposition is reset
to the value it had when the shell was started.
If arg is the null string, then the signal specified by
each sigspec is ignored by the shell and commands it invokes.
If arg is not present and -p has been supplied,
the shell displays the trap commands associated with each sigspec.
If no arguments are supplied, or
only -p is given, trap prints the list of commands
associated with each signal number in a form that may be reused as
shell input.
The -l option causes the shell to print a list of signal names
and their corresponding numbers.
Each sigspec is either a signal name or a signal number.
Signal names are case insensitive and the SIG prefix is optional.

If a sigspec
is 0 or EXIT, arg is executed when the shell exits.
If a sigspec is DEBUG, the command arg is executed
before every simple command, for command, case command,
select command, every arithmetic for command, and before
the first command executes in a shell function.
Refer to the description of the extdebug option to the
shopt builtin (see The Shopt Builtin) for details of its
effect on the DEBUG trap.
If a sigspec is RETURN, the command arg is executed
each time a shell function or a script executed with the . or
source builtins finishes executing.

If a sigspec is ERR, the command arg
is executed whenever
a pipeline (which may consist of a single simple
command), a list, or a compound command returns a
non-zero exit status,
subject to the following conditions.
The ERR trap is not executed if the failed command is part of the
command list immediately following an until or while keyword,
part of the test following the if or elif reserved words,
part of a command executed in a && or || list
except the command following the final && or ||,
any command in a pipeline but the last,
or if the command’s return
status is being inverted using !.
These are the same conditions obeyed by the errexit (-e)
option.

Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals that are not being ignored are reset to their original
values in a subshell or subshell environment when one is created.

The return status is zero unless a sigspec does not specify a
valid signal.

umask

umask [-p] [-S] [mode]

Set the shell process’s file creation mask to mode. If
mode begins with a digit, it is interpreted as an octal number;
if not, it is interpreted as a symbolic mode mask similar
to that accepted by the chmod command. If mode is
omitted, the current value of the mask is printed. If the -S
option is supplied without a mode argument, the mask is printed
in a symbolic format.
If the -p option is supplied, and mode
is omitted, the output is in a form that may be reused as input.
The return status is zero if the mode is successfully changed or if
no mode argument is supplied, and non-zero otherwise.

Note that when the mode is interpreted as an octal number, each number
of the umask is subtracted from 7. Thus, a umask of 022
results in permissions of 755.

unset

unset [-fnv] [name]

Remove each variable or function name.
If the -v option is given, each
name refers to a shell variable and that variable is removed.
If the -f option is given, the names refer to shell
functions, and the function definition is removed.
If the -n option is supplied, and name is a variable with
the nameref attribute, name will be unset rather than the
variable it references.
-n has no effect if the -f option is supplied.
If no options are supplied, each name refers to a variable; if
there is no variable by that name, any function with that name is
unset.
Readonly variables and functions may not be unset.
The return status is zero unless a name is readonly.

4.2 Bash Builtin Commands

This section describes builtin commands which are unique to
or have been extended in Bash.
Some of these commands are specified in the POSIX standard.

alias

alias [-p] [name[=value] …]

Without arguments or with the -p option, alias prints
the list of aliases on the standard output in a form that allows
them to be reused as input.
If arguments are supplied, an alias is defined for each name
whose value is given. If no value is given, the name
and value of the alias is printed.
Aliases are described in Aliases.

Display current Readline (see Command Line Editing)
key and function bindings,
bind a key sequence to a Readline function or macro,
or set a Readline variable.
Each non-option argument is a command as it would appear in a
Readline initialization file (see Readline Init File),
but each binding or command must be passed as a separate argument; e.g.,
‘"\C-x\C-r":re-read-init-file’.

Options, if supplied, have the following meanings:

-m keymap

Use keymap as the keymap to be affected by
the subsequent bindings. Acceptable keymap
names are
emacs,
emacs-standard,
emacs-meta,
emacs-ctlx,
vi,
vi-move,
vi-command, and
vi-insert.
vi is equivalent to vi-command (vi-move is also a
synonym); emacs is equivalent to emacs-standard.

-l

List the names of all Readline functions.

-p

Display Readline function names and bindings in such a way that they
can be used as input or in a Readline initialization file.

-P

List current Readline function names and bindings.

-v

Display Readline variable names and values in such a way that they
can be used as input or in a Readline initialization file.

-V

List current Readline variable names and values.

-s

Display Readline key sequences bound to macros and the strings they output
in such a way that they can be used as input or in a Readline
initialization file.

-S

Display Readline key sequences bound to macros and the strings they output.

-f filename

Read key bindings from filename.

-q function

Query about which keys invoke the named function.

-u function

Unbind all keys bound to the named function.

-r keyseq

Remove any current binding for keyseq.

-x keyseq:shell-command

Cause shell-command to be executed whenever keyseq is
entered.
When shell-command is executed, the shell sets the
READLINE_LINE variable to the contents of the Readline line
buffer and the READLINE_POINT variable to the current location
of the insertion point.
If the executed command changes the value of READLINE_LINE or
READLINE_POINT, those new values will be reflected in the
editing state.

-X

List all key sequences bound to shell commands and the associated commands
in a format that can be reused as input.

The return status is zero unless an invalid option is supplied or an
error occurs.

builtin

builtin [shell-builtin [args]]

Run a shell builtin, passing it args, and return its exit status.
This is useful when defining a shell function with the same
name as a shell builtin, retaining the functionality of the builtin within
the function.
The return status is non-zero if shell-builtin is not a shell
builtin command.

caller

caller [expr]

Returns the context of any active subroutine call (a shell function or
a script executed with the . or source builtins).

Without expr, caller displays the line number and source
filename of the current subroutine call.
If a non-negative integer is supplied as expr, caller
displays the line number, subroutine name, and source file corresponding
to that position in the current execution call stack. This extra
information may be used, for example, to print a stack trace. The
current frame is frame 0.

The return value is 0 unless the shell is not executing a subroutine
call or expr does not correspond to a valid position in the
call stack.

command

command [-pVv] command [arguments …]

Runs command with arguments ignoring any shell function
named command.
Only shell builtin commands or commands found by searching the
PATH are executed.
If there is a shell function named ls, running ‘command ls’
within the function will execute the external command ls
instead of calling the function recursively.
The -p option means to use a default value for PATH
that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if command cannot be
found or an error occurred, and the exit status of command
otherwise.

If either the -V or -v option is supplied, a
description of command is printed. The -v option
causes a single word indicating the command or file name used to
invoke command to be displayed; the -V option produces
a more verbose description. In this case, the return status is
zero if command is found, and non-zero if not.

declare

declare [-aAfFgilnrtux] [-p] [name[=value] …]

Declare variables and give them attributes. If no names
are given, then display the values of variables instead.

The -p option will display the attributes and values of each
name.
When -p is used with name arguments, additional options,
other than -f and -F, are ignored.

When -p is supplied without name arguments, declare
will display the attributes and values of all variables having the
attributes specified by the additional options.
If no other options are supplied with -p, declare will
display the attributes and values of all shell variables. The -f
option will restrict the display to shell functions.

The -F option inhibits the display of function definitions;
only the function name and attributes are printed.
If the extdebug shell option is enabled using shopt
(see The Shopt Builtin), the source file name and line number where
each name is defined are displayed as well.
-F implies -f.

The -g option forces variables to be created or modified at
the global scope, even when declare is executed in a shell function.
It is ignored in all other cases.

The following options can be used to restrict output to variables with
the specified attributes or to give variables attributes:

The variable is to be treated as
an integer; arithmetic evaluation (see Shell Arithmetic) is
performed when the variable is assigned a value.

-l

When the variable is assigned a value, all upper-case characters are
converted to lower-case.
The upper-case attribute is disabled.

-n

Give each name the nameref attribute, making
it a name reference to another variable.
That other variable is defined by the value of name.
All references, assignments, and attribute modifications
to name, except for those using or changing the
-n attribute itself, are performed on the variable referenced by
name’s value.
The nameref attribute cannot be applied to array variables.

-r

Make names readonly. These names cannot then be assigned values
by subsequent assignment statements or unset.

-t

Give each name the trace attribute.
Traced functions inherit the DEBUG and RETURN traps from
the calling shell.
The trace attribute has no special meaning for variables.

-u

When the variable is assigned a value, all lower-case characters are
converted to upper-case.
The lower-case attribute is disabled.

-x

Mark each name for export to subsequent commands via
the environment.

Using ‘+’ instead of ‘-’ turns off the attribute instead,
with the exceptions that ‘+a’ and ‘+A’
may not be used to destroy array variables and ‘+r’ will not
remove the readonly attribute.
When used in a function, declare makes each name local,
as with the local command, unless the -g option is used.
If a variable name is followed by =value, the value of the variable
is set to value.

When using -a or -A and the compound assignment syntax to
create array variables, additional attributes do not take effect until
subsequent assignments.

The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using ‘-f foo=bar’,
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without
using the compound assignment syntax (see Arrays),
one of the names is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with -f.

echo

echo [-neE] [arg …]

Output the args, separated by spaces, terminated with a
newline.
The return status is 0 unless a write error occurs.
If -n is specified, the trailing newline is suppressed.
If the -e option is given, interpretation of the following
backslash-escaped characters is enabled.
The -E option disables the interpretation of these escape characters,
even on systems where they are interpreted by default.
The xpg_echo shell option may be used to
dynamically determine whether or not echo expands these
escape characters by default.
echo does not interpret -- to mean the end of options.

echo interprets the following escape sequences:

\a

alert (bell)

\b

backspace

\c

suppress further output

\e

\E

escape

\f

form feed

\n

new line

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\0nnn

the eight-bit character whose value is the octal value nnn
(zero to three octal digits)

\xHH

the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)

\uHHHH

the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH (one to four hex digits)

Enable and disable builtin shell commands.
Disabling a builtin allows a disk command which has the same name
as a shell builtin to be executed without specifying a full pathname,
even though the shell normally searches for builtins before disk commands.
If -n is used, the names become disabled. Otherwise
names are enabled. For example, to use the test binary
found via $PATH instead of the shell builtin version, type
‘enable -n test’.

If the -p option is supplied, or no name arguments appear,
a list of shell builtins is printed. With no other arguments, the list
consists of all enabled shell builtins.
The -a option means to list
each builtin with an indication of whether or not it is enabled.

The -f option means to load the new builtin command name
from shared object filename, on systems that support dynamic loading.
The -d option will delete a builtin loaded with -f.

If there are no options, a list of the shell builtins is displayed.
The -s option restricts enable to the POSIX special
builtins. If -s is used with -f, the new builtin becomes
a special builtin (see Special Builtins).

The return status is zero unless a name is not a shell builtin
or there is an error loading a new builtin from a shared object.

help

help [-dms] [pattern]

Display helpful information about builtin commands.
If pattern is specified, help gives detailed help
on all commands matching pattern, otherwise a list of
the builtins is printed.

Options, if supplied, have the following meanings:

-d

Display a short description of each pattern

-m

Display the description of each pattern in a manpage-like format

-s

Display only a short usage synopsis for each pattern

The return status is zero unless no command matches pattern.

let

let expression [expression …]

The let builtin allows arithmetic to be performed on shell
variables. Each expression is evaluated according to the
rules given below in Shell Arithmetic. If the
last expression evaluates to 0, let returns 1;
otherwise 0 is returned.

local

local [option] name[=value] …

For each argument, a local variable named name is created,
and assigned value.
The option can be any of the options accepted by declare.
local can only be used within a function; it makes the variable
name have a visible scope restricted to that function and its
children.
If name is ‘-’, the set of shell options is made local to the
function in which local is invoked: shell options changed using
the set builtin inside the function are restored to their original
values when the function returns.
The return status is zero unless local is used outside
a function, an invalid name is supplied, or name is a
readonly variable.

Read lines from the standard input into the indexed array variable array,
or from file descriptor fd
if the -u option is supplied.
The variable MAPFILE is the default array.
Options, if supplied, have the following meanings:

-d

The first character of delim is used to terminate each input line,
rather than newline.
If delim is the empty string, mapfile will terminate a line
when it reads a NUL character.

-n

Copy at most count lines. If count is 0, all lines are copied.

-O

Begin assigning to array at index origin.
The default index is 0.

-s

Discard the first count lines read.

-t

Remove a trailing delim (default newline) from each line read.

-u

Read lines from file descriptor fd instead of the standard input.

-C

Evaluate callback each time quantum lines are read.
The -c option specifies quantum.

-c

Specify the number of lines read between each call to callback.

If -C is specified without -c,
the default quantum is 5000.
When callback is evaluated, it is supplied the index of the next
array element to be assigned and the line to be assigned to that element
as additional arguments.
callback is evaluated after the line is read but before the
array element is assigned.

If not supplied with an explicit origin, mapfile will clear array
before assigning to it.

mapfile returns successfully unless an invalid option or option
argument is supplied, array is invalid or unassignable, or array
is not an indexed array.

printf

printf [-v var] format [arguments]

Write the formatted arguments to the standard output under the
control of the format.
The -v option causes the output to be assigned to the variable
var rather than being printed to the standard output.

The format is a character string which contains three types of objects:
plain characters, which are simply copied to standard output, character
escape sequences, which are converted and copied to the standard output, and
format specifications, each of which causes printing of the next successive
argument.
In addition to the standard printf(1) formats, printf
interprets the following extensions:

%b

Causes printf to expand backslash escape sequences in the
corresponding argument in the same way as echo -e
(see Bash Builtins).

%q

Causes printf to output the
corresponding argument in a format that can be reused as shell input.

%(datefmt)T

Causes printf to output the date-time string resulting from using
datefmt as a format string for strftime(3).
The corresponding argument is an integer representing the number of
seconds since the epoch.
Two special argument values may be used: -1 represents the current
time, and -2 represents the time the shell was invoked.
If no argument is specified, conversion behaves as if -1 had been given.
This is an exception to the usual printf behavior.

Arguments to non-string format specifiers are treated as C language constants,
except that a leading plus or minus sign is allowed, and if the leading
character is a single or double quote, the value is the ASCII value of
the following character.

The format is reused as necessary to consume all of the arguments.
If the format requires more arguments than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied. The return value is zero on success,
non-zero on failure.

One line is read from the standard input, or from the file descriptor
fd supplied as an argument to the -u option,
split into words as described above in Word Splitting,
and the first word
is assigned to the first name, the second word to the second name,
and so on.
If there are more words than names,
the remaining words and their intervening delimiters are assigned
to the last name.
If there are fewer words read from the input stream than names,
the remaining names are assigned empty values.
The characters in the value of the IFS variable
are used to split the line into words using the same rules the shell
uses for expansion (described above in Word Splitting).
The backslash character ‘\’ may be used to remove any special
meaning for the next character read and for line continuation.
If no names are supplied, the line read is assigned to the
variable REPLY.
The exit status is zero, unless end-of-file is encountered, read
times out (in which case the status is greater than 128),
a variable assignment error (such as assigning to a readonly variable) occurs,
or an invalid file descriptor is supplied as the argument to -u.

Options, if supplied, have the following meanings:

-a aname

The words are assigned to sequential indices of the array variable
aname, starting at 0.
All elements are removed from aname before the assignment.
Other name arguments are ignored.

-d delim

The first character of delim is used to terminate the input line,
rather than newline.
If delim is the empty string, read will terminate a line
when it reads a NUL character.

-e

Readline (see Command Line Editing) is used to obtain the line.
Readline uses the current (or default, if line editing was not previously
active) editing settings, but uses Readline’s default filename completion.

-i text

If Readline is being used to read the line, text is placed into
the editing buffer before editing begins.

-n nchars

read returns after reading nchars characters rather than
waiting for a complete line of input, but honors a delimiter if fewer
than nchars characters are read before the delimiter.

-N nchars

read returns after reading exactly nchars characters rather
than waiting for a complete line of input, unless EOF is encountered or
read times out.
Delimiter characters encountered in the input are
not treated specially and do not cause read to return until
nchars characters are read.
The result is not split on the characters in IFS; the intent is
that the variable is assigned exactly the characters read
(with the exception of backslash; see the -r option below).

-p prompt

Display prompt, without a trailing newline, before attempting
to read any input.
The prompt is displayed only if input is coming from a terminal.

-r

If this option is given, backslash does not act as an escape character.
The backslash is considered to be part of the line.
In particular, a backslash-newline pair may not then be used as a line
continuation.

-s

Silent mode. If input is coming from a terminal, characters are
not echoed.

-t timeout

Cause read to time out and return failure if a complete line of
input (or a specified number of characters)
is not read within timeout seconds.
timeout may be a decimal number with a fractional portion following
the decimal point.
This option is only effective if read is reading input from a
terminal, pipe, or other special file; it has no effect when reading
from regular files.
If read times out, read saves any partial input read into
the specified variable name.
If timeout is 0, read returns immediately, without trying to
read and data. The exit status is 0 if input is available on
the specified file descriptor, non-zero otherwise.
The exit status is greater than 128 if the timeout is exceeded.

For each name, indicate how it would be interpreted if used as a
command name.

If the -t option is used, type prints a single word
which is one of ‘alias’, ‘function’, ‘builtin’,
‘file’ or ‘keyword’,
if name is an alias, shell function, shell builtin,
disk file, or shell reserved word, respectively.
If the name is not found, then nothing is printed, and
type returns a failure status.

If the -p option is used, type either returns the name
of the disk file that would be executed, or nothing if -t
would not return ‘file’.

The -P option forces a path search for each name, even if
-t would not return ‘file’.

If a command is hashed, -p and -P print the hashed value,
which is not necessarily the file that appears first in $PATH.

If the -a option is used, type returns all of the places
that contain an executable named file.
This includes aliases and functions, if and only if the -p option
is not also used.

If the -f option is used, type does not attempt to find
shell functions, as with the command builtin.

The return status is zero if all of the names are found, non-zero
if any are not found.

typeset

typeset [-afFgrxilnrtux] [-p] [name[=value] …]

The typeset command is supplied for compatibility with the Korn
shell.
It is a synonym for the declare builtin command.

ulimit

ulimit [-HSabcdefiklmnpqrstuvxPT] [limit]

ulimit provides control over the resources available to processes
started by the shell, on systems that allow such control. If an
option is given, it is interpreted as follows:

-S

Change and report the soft limit associated with a resource.

-H

Change and report the hard limit associated with a resource.

-a

All current limits are reported.

-b

The maximum socket buffer size.

-c

The maximum size of core files created.

-d

The maximum size of a process’s data segment.

-e

The maximum scheduling priority ("nice").

-f

The maximum size of files written by the shell and its children.

-i

The maximum number of pending signals.

-k

The maximum number of kqueues that may be allocated.

-l

The maximum size that may be locked into memory.

-m

The maximum resident set size (many systems do not honor this limit).

-n

The maximum number of open file descriptors (most systems do not
allow this value to be set).

-p

The pipe buffer size.

-q

The maximum number of bytes in POSIX message queues.

-r

The maximum real-time scheduling priority.

-s

The maximum stack size.

-t

The maximum amount of cpu time in seconds.

-u

The maximum number of processes available to a single user.

-v

The maximum amount of virtual memory available to the shell, and, on
some systems, to its children.

-x

The maximum number of file locks.

-P

The maximum number of pseudoterminals.

-T

The maximum number of threads.

If limit is given, and the -a option is not used,
limit is the new value of the specified resource.
The special limit values hard, soft, and
unlimited stand for the current hard limit, the current soft limit,
and no limit, respectively.
A hard limit cannot be increased by a non-root user once it is set;
a soft limit may be increased up to the value of the hard limit.
Otherwise, the current value of the soft limit for the specified resource
is printed, unless the -H option is supplied.
When setting new limits, if neither -H nor -S is supplied,
both the hard and soft limits are set.
If no option is given, then -f is assumed. Values are in 1024-byte
increments, except for -t, which is in seconds; -p,
which is in units of 512-byte blocks;
-P,
-T,
-b,
-k,
-n and -u, which are unscaled values;
and, when in POSIX Mode (see Bash POSIX Mode),
-c and -f, which are in 512-byte increments.

The return status is zero unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.

unalias

unalias [-a] [name … ]

Remove each name from the list of aliases. If -a is
supplied, all aliases are removed.
Aliases are described in Aliases.

4.3.1 The Set Builtin

This builtin is so complicated that it deserves its own section. set
allows you to change the values of shell options and set the positional
parameters, or to display the names and values of shell variables.

If no options or arguments are supplied, set displays the names
and values of all shell variables and functions, sorted according to the
current locale, in a format that may be reused as input
for setting or resetting the currently-set variables.
Read-only variables cannot be reset.
In POSIX mode, only shell variables are listed.

When options are supplied, they set or unset shell attributes.
Options, if specified, have the following meanings:

-a

Each variable or function that is created or modified is given the
export attribute and marked for export to the environment of
subsequent commands.

-b

Cause the status of terminated background jobs to be reported
immediately, rather than before printing the next primary prompt.

-e

Exit immediately if
a pipeline (see Pipelines), which may consist of a single simple command
(see Simple Commands),
a list (see Lists),
or a compound command (see Compound Commands)
returns a non-zero status.
The shell does not exit if the command that fails is part of the
command list immediately following a while or until keyword,
part of the test in an if statement,
part of any command executed in a && or || list except
the command following the final && or ||,
any command in a pipeline but the last,
or if the command’s return status is being inverted with !.
If a compound command other than a subshell
returns a non-zero status because a command failed
while -e was being ignored, the shell does not exit.
A trap on ERR, if set, is executed before the shell exits.

This option applies to the shell environment and each subshell environment
separately (see Command Execution Environment), and may cause
subshells to exit before executing all the commands in the subshell.

If a compound command or shell function executes in a context where
-e is being ignored,
none of the commands executed within the compound command or function body
will be affected by the -e setting, even if -e is set
and a command returns a failure status.
If a compound command or shell function sets -e while executing in
a context where -e is ignored, that setting will not have any
effect until the compound command or the command containing the function
call completes.

-f

Disable filename expansion (globbing).

-h

Locate and remember (hash) commands as they are looked up for execution.
This option is enabled by default.

-k

All arguments in the form of assignment statements are placed
in the environment for a command, not just those that precede
the command name.

-m

Job control is enabled (see Job Control).
All processes run in a separate process group.
When a background job completes, the shell prints a line
containing its exit status.

-n

Read commands but do not execute them.
This may be used to check a script for syntax errors.
This option is ignored by interactive shells.

-o option-name

Set the option corresponding to option-name:

allexport

Same as -a.

braceexpand

Same as -B.

emacs

Use an emacs-style line editing interface (see Command Line Editing).
This also affects the editing interface used for read -e.

errexit

Same as -e.

errtrace

Same as -E.

functrace

Same as -T.

hashall

Same as -h.

histexpand

Same as -H.

history

Enable command history, as described in Bash History Facilities.
This option is on by default in interactive shells.

ignoreeof

An interactive shell will not exit upon reading EOF.

keyword

Same as -k.

monitor

Same as -m.

noclobber

Same as -C.

noexec

Same as -n.

noglob

Same as -f.

nolog

Currently ignored.

notify

Same as -b.

nounset

Same as -u.

onecmd

Same as -t.

physical

Same as -P.

pipefail

If set, the return value of a pipeline is the value of the last
(rightmost) command to exit with a non-zero status, or zero if all
commands in the pipeline exit successfully.
This option is disabled by default.

posix

Change the behavior of Bash where the default operation differs
from the POSIX standard to match the standard
(see Bash POSIX Mode).
This is intended to make Bash behave as a strict superset of that
standard.

privileged

Same as -p.

verbose

Same as -v.

vi

Use a vi-style line editing interface.
This also affects the editing interface used for read -e.

xtrace

Same as -x.

-p

Turn on privileged mode.
In this mode, the $BASH_ENV and $ENV files are not
processed, shell functions are not inherited from the environment,
and the SHELLOPTS, BASHOPTS, CDPATH and GLOBIGNORE
variables, if they appear in the environment, are ignored.
If the shell is started with the effective user (group) id not equal to the
real user (group) id, and the -p option is not supplied, these actions
are taken and the effective user id is set to the real user id.
If the -p option is supplied at startup, the effective user id is
not reset.
Turning this option off causes the effective user
and group ids to be set to the real user and group ids.

-t

Exit after reading and executing one command.

-u

Treat unset variables and parameters other than the special parameters
‘@’ or ‘*’ as an error when performing parameter expansion.
An error message will be written to the standard error, and a non-interactive
shell will exit.

-v

Print shell input lines as they are read.

-x

Print a trace of simple commands, for commands, case
commands, select commands, and arithmetic for commands
and their arguments or associated word lists after they are
expanded and before they are executed. The value of the PS4
variable is expanded and the resultant value is printed before
the command and its expanded arguments.

-B

The shell will perform brace expansion (see Brace Expansion).
This option is on by default.

If set, any trap on ERR is inherited by shell functions, command
substitutions, and commands executed in a subshell environment.
The ERR trap is normally not inherited in such cases.

-H

Enable ‘!’ style history substitution (see History Interaction).
This option is on by default for interactive shells.

-P

If set, do not resolve symbolic links when performing commands such as
cd which change the current directory. The physical directory
is used instead. By default, Bash follows
the logical chain of directories when performing commands
which change the current directory.

For example, if /usr/sys is a symbolic link to /usr/local/sys
then:

$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr

If set -P is on, then:

$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local

-T

If set, any trap on DEBUG and RETURN are inherited by
shell functions, command substitutions, and commands executed
in a subshell environment.
The DEBUG and RETURN traps are normally not inherited
in such cases.

--

If no arguments follow this option, then the positional parameters are
unset. Otherwise, the positional parameters are set to the
arguments, even if some of them begin with a ‘-’.

-

Signal the end of options, cause all remaining arguments
to be assigned to the positional parameters. The -x
and -v options are turned off.
If there are no arguments, the positional parameters remain unchanged.

Using ‘+’ rather than ‘-’ causes these options to be
turned off. The options can also be used upon invocation of the
shell. The current set of options may be found in $-.

The remaining N arguments are positional parameters and are
assigned, in order, to $1, $2, … $N.
The special parameter # is set to N.

The return status is always zero unless an invalid option is supplied.

4.3.2 The Shopt Builtin

This builtin allows you to change additional shell optional behavior.

shopt

shopt [-pqsu] [-o] [optname …]

Toggle the values of settings controlling optional shell behavior.
The settings can be either those listed below, or, if the
-o option is used, those available with the -o
option to the set builtin command (see The Set Builtin).
With no options, or with the -p option, a list of all settable
options is displayed, with an indication of whether or not each is set;
if optnames are supplied, the output is restricted to those options.
The -p option causes output to be displayed in a form that
may be reused as input.
Other options have the following meanings:

-s

Enable (set) each optname.

-u

Disable (unset) each optname.

-q

Suppresses normal output; the return status
indicates whether the optname is set or unset.
If multiple optname arguments are given with -q,
the return status is zero if all optnames are enabled;
non-zero otherwise.

-o

Restricts the values of
optname to be those defined for the -o option to the
set builtin (see The Set Builtin).

If either -s or -u
is used with no optname arguments, shopt shows only
those options which are set or unset, respectively.

The return status when listing options is zero if all optnames
are enabled, non-zero otherwise. When setting or unsetting options,
the return status is zero unless an optname is not a valid shell
option.

The list of shopt options is:

assoc_expand_once

If set, the shell suppresses multiple evaluation of associative array
subscripts during arithmetic expression evaluation, while executing
builtins that can perform variable assignments,
and while executing builtins that perform array dereferencing.

autocd

If set, a command name that is the name of a directory is executed as if
it were the argument to the cd command.
This option is only used by interactive shells.

cdable_vars

If this is set, an argument to the cd builtin command that
is not a directory is assumed to be the name of a variable whose
value is the directory to change to.

cdspell

If set, minor errors in the spelling of a directory component in a
cd command will be corrected.
The errors checked for are transposed characters,
a missing character, and a character too many.
If a correction is found, the corrected path is printed,
and the command proceeds.
This option is only used by interactive shells.

checkhash

If this is set, Bash checks that a command found in the hash
table exists before trying to execute it. If a hashed command no
longer exists, a normal path search is performed.

checkjobs

If set, Bash lists the status of any stopped and running jobs before
exiting an interactive shell. If any jobs are running, this causes
the exit to be deferred until a second exit is attempted without an
intervening command (see Job Control).
The shell always postpones exiting if any jobs are stopped.

checkwinsize

If set, Bash checks the window size after each external (non-builtin)
command and, if necessary, updates the values of
LINES and COLUMNS.
This option is enabled by default.

cmdhist

If set, Bash
attempts to save all lines of a multiple-line
command in the same history entry. This allows
easy re-editing of multi-line commands.
This option is enabled by default, but only has an effect if command
history is enabled (see Bash History Facilities).

compat31

If set, Bash
changes its behavior to that of version 3.1 with respect to quoted
arguments to the conditional command’s ‘=~’ operator
and with respect to locale-specific
string comparison when using the [[
conditional command’s ‘<’ and ‘>’ operators.
Bash versions prior to bash-4.1 use ASCII collation and strcmp(3);
bash-4.1 and later use the current locale’s collation sequence and strcoll(3).

compat32

If set, Bash
changes its behavior to that of version 3.2 with respect to locale-specific
string comparison when using the [[
conditional command’s ‘<’ and ‘>’ operators (see previous item)
and the effect of interrupting a command list.
Bash versions 3.2 and earlier continue with the next command in the list
after one terminates due to an interrupt.

compat40

If set, Bash
changes its behavior to that of version 4.0 with respect to locale-specific
string comparison when using the [[
conditional command’s ‘<’ and ‘>’ operators (see description
of compat31)
and the effect of interrupting a command list.
Bash versions 4.0 and later interrupt the list as if the shell received the
interrupt; previous versions continue with the next command in the list.

compat41

If set, Bash, when in POSIX mode, treats a single quote in a double-quoted
parameter expansion as a special character. The single quotes must match
(an even number) and the characters between the single quotes are considered
quoted. This is the behavior of POSIX mode through version 4.1.
The default Bash behavior remains as in previous versions.

compat42

If set, Bash
does not process the replacement string in the pattern substitution word
expansion using quote removal.

compat43

If set, Bash
does not print a warning message if an attempt is made to use a quoted compound
array assignment as an argument to declare,
makes word expansion errors
non-fatal errors that cause the current command to fail (the default behavior is
to make them fatal errors that cause the shell to exit),
and does not reset the
loop state when a shell function is executed (this allows break or
continue in a shell function to affect loops in the caller’s context).

compat44

If set, Bash
saves the positional parameters to BASH_ARGV and BASH_ARGC before they are
used, regardless of whether or not extended debugging mode is enabled.

complete_fullquote

If set, Bash
quotes all shell metacharacters in filenames and directory names when
performing completion.
If not set, Bash
removes metacharacters such as the dollar sign from the set of
characters that will be quoted in completed filenames
when these metacharacters appear in shell variable references in words to be
completed.
This means that dollar signs in variable names that expand to directories
will not be quoted;
however, any dollar signs appearing in filenames will not be quoted, either.
This is active only when bash is using backslashes to quote completed
filenames.
This variable is set by default, which is the default Bash behavior in
versions through 4.2.

direxpand

If set, Bash
replaces directory names with the results of word expansion when performing
filename completion. This changes the contents of the readline editing
buffer.
If not set, Bash attempts to preserve what the user typed.

dirspell

If set, Bash
attempts spelling correction on directory names during word completion
if the directory name initially supplied does not exist.

dotglob

If set, Bash includes filenames beginning with a ‘.’ in
the results of filename expansion.
The filenames ‘.’ and ‘..’ must always be matched explicitly,
even if dotglob is set.

execfail

If this is set, a non-interactive shell will not exit if
it cannot execute the file specified as an argument to the exec
builtin command. An interactive shell does not exit if exec
fails.

expand_aliases

If set, aliases are expanded as described below under Aliases,
Aliases.
This option is enabled by default for interactive shells.

extdebug

If set at shell invocation,
or in a shell startup file,
arrange to execute the debugger profile
before the shell starts, identical to the --debugger option.
If set after invocation, behavior intended for use by debuggers is enabled:

The -F option to the declare builtin (see Bash Builtins)
displays the source file name and line number corresponding to each function
name supplied as an argument.

If the command run by the DEBUG trap returns a non-zero value, the
next command is skipped and not executed.

If the command run by the DEBUG trap returns a value of 2, and the
shell is executing in a subroutine (a shell function or a shell script
executed by the . or source builtins), the shell simulates
a call to return.

BASH_ARGC and BASH_ARGV are updated as described in their
descriptions (see Bash Variables).

If set, the extended pattern matching features described above
(see Pattern Matching) are enabled.

extquote

If set, $'string' and $"string" quoting is
performed within ${parameter} expansions
enclosed in double quotes. This option is enabled by default.

failglob

If set, patterns which fail to match filenames during filename expansion
result in an expansion error.

force_fignore

If set, the suffixes specified by the FIGNORE shell variable
cause words to be ignored when performing word completion even if
the ignored words are the only possible completions.
See Bash Variables, for a description of FIGNORE.
This option is enabled by default.

globasciiranges

If set, range expressions used in pattern matching bracket expressions
(see Pattern Matching)
behave as if in the traditional C locale when performing
comparisons. That is, the current locale’s collating sequence
is not taken into account, so
‘b’ will not collate between ‘A’ and ‘B’,
and upper-case and lower-case ASCII characters will collate together.

globstar

If set, the pattern ‘**’ used in a filename expansion context will
match all files and zero or more directories and subdirectories.
If the pattern is followed by a ‘/’, only directories and
subdirectories match.

gnu_errfmt

If set, shell error messages are written in the standard GNU error
message format.

histappend

If set, the history list is appended to the file named by the value
of the HISTFILE
variable when the shell exits, rather than overwriting the file.

histreedit

If set, and Readline
is being used, a user is given the opportunity to re-edit a
failed history substitution.

histverify

If set, and Readline
is being used, the results of history substitution are not immediately
passed to the shell parser. Instead, the resulting line is loaded into
the Readline editing buffer, allowing further modification.

hostcomplete

If set, and Readline is being used, Bash will attempt to perform
hostname completion when a word containing a ‘@’ is being
completed (see Commands For Completion). This option is enabled
by default.

huponexit

If set, Bash will send SIGHUP to all jobs when an interactive
login shell exits (see Signals).

inherit_errexit

If set, command substitution inherits the value of the errexit option,
instead of unsetting it in the subshell environment.
This option is enabled when POSIX mode is enabled.

interactive_comments

Allow a word beginning with ‘#’
to cause that word and all remaining characters on that
line to be ignored in an interactive shell.
This option is enabled by default.

lastpipe

If set, and job control is not active, the shell runs the last command of
a pipeline not executed in the background in the current shell environment.

lithist

If enabled, and the cmdhist
option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.

localvar_inherit

If set, local variables inherit the value and attributes of a variable of
the same name that exists at a previous scope before any new value is
assigned. The nameref attribute is not inherited.

localvar_unset

If set, calling unset on local variables in previous function scopes
marks them so subsequent lookups find them unset until that function
returns. This is identical to the behavior of unsetting local variables
at the current function scope.

login_shell

The shell sets this option if it is started as a login shell
(see Invoking Bash).
The value may not be changed.

mailwarn

If set, and a file that Bash is checking for mail has been
accessed since the last time it was checked, the message
"The mail in mailfile has been read" is displayed.

no_empty_cmd_completion

If set, and Readline is being used, Bash will not attempt to search
the PATH for possible completions when completion is attempted
on an empty line.

nocaseglob

If set, Bash matches filenames in a case-insensitive fashion when
performing filename expansion.

nocasematch

If set, Bash matches patterns in a case-insensitive fashion when
performing matching while executing case or [[
conditional commands,
when performing pattern substitution word expansions,
or when filtering possible completions as part of programmable completion.

nullglob

If set, Bash allows filename patterns which match no
files to expand to a null string, rather than themselves.

progcomp

If set, the programmable completion facilities
(see Programmable Completion) are enabled.
This option is enabled by default.

progcomp_alias

If set, and programmable completion is enabled, Bash treats a command
name that doesn’t have any completions as a possible alias and attempts
alias expansion. If it has an alias, Bash attempts programmable
completion using the command word resulting from the expanded alias.

promptvars

If set, prompt strings undergo
parameter expansion, command substitution, arithmetic
expansion, and quote removal after being expanded
as described below (see Controlling the Prompt).
This option is enabled by default.

restricted_shell

The shell sets this option if it is started in restricted mode
(see The Restricted Shell).
The value may not be changed.
This is not reset when the startup files are executed, allowing
the startup files to discover whether or not a shell is restricted.

shift_verbose

If this is set, the shift
builtin prints an error message when the shift count exceeds the
number of positional parameters.

sourcepath

If set, the source builtin uses the value of PATH
to find the directory containing the file supplied as an argument.
This option is enabled by default.

xpg_echo

If set, the echo builtin expands backslash-escape sequences
by default.

The return status when listing options is zero if all optnames
are enabled, non-zero otherwise.
When setting or unsetting options, the return status is zero unless an
optname is not a valid shell option.

4.4 Special Builtins

For historical reasons, the POSIX standard has classified
several builtin commands as special.
When Bash is executing in POSIX mode, the special builtins
differ from other builtin commands in three respects:

Special builtins are found before shell functions during command lookup.

If a special builtin returns an error status, a non-interactive shell exits.

Assignment statements preceding the command stay in effect in the shell
environment after the command completes.

When Bash is not executing in POSIX mode, these builtins behave no
differently than the rest of the Bash builtin commands.
The Bash POSIX mode is described in Bash POSIX Mode.

5.1 Bourne Shell Variables

Bash uses certain shell variables in the same way as the Bourne shell.
In some cases, Bash assigns a default value to the variable.

CDPATH

A colon-separated list of directories used as a search path for
the cd builtin command.

HOME

The current user’s home directory; the default for the cd builtin
command.
The value of this variable is also used by tilde expansion
(see Tilde Expansion).

IFS

A list of characters that separate fields; used when the shell splits
words as part of expansion.

MAIL

If this parameter is set to a filename or directory name
and the MAILPATH variable
is not set, Bash informs the user of the arrival of mail in
the specified file or Maildir-format directory.

MAILPATH

A colon-separated list of filenames which the shell periodically checks
for new mail.
Each list entry can specify the message that is printed when new mail
arrives in the mail file by separating the filename from the message with
a ‘?’.
When used in the text of the message, $_ expands to the name of
the current mail file.

OPTARG

The value of the last option argument processed by the getopts builtin.

OPTIND

The index of the last option argument processed by the getopts builtin.

PATH

A colon-separated list of directories in which the shell looks for
commands.
A zero-length (null) directory name in the value of PATH indicates the
current directory.
A null directory name may appear as two adjacent colons, or as an initial
or trailing colon.

PS1

The primary prompt string. The default value is ‘\s-\v\$ ’.
See Controlling the Prompt, for the complete list of escape
sequences that are expanded before PS1 is displayed.

PS2

The secondary prompt string. The default value is ‘> ’.
PS2 is expanded in the same way as PS1 before being
displayed.

5.2 Bash Variables

These variables are set or used by Bash, but other shells
do not normally treat them specially.

A few variables used by Bash are described in different chapters:
variables for controlling the job control facilities
(see Job Control Variables).

BASH

The full pathname used to execute the current instance of Bash.

BASHOPTS

A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the -s option to the
shopt builtin command (see The Shopt Builtin).
The options appearing in BASHOPTS are those reported
as ‘on’ by ‘shopt’.
If this variable is in the environment when Bash
starts up, each shell option in the list will be enabled before
reading any startup files. This variable is readonly.

BASHPID

Expands to the process ID of the current Bash process.
This differs from $$ under certain circumstances, such as subshells
that do not require Bash to be re-initialized.
Assignments to BASHPID have no effect.
If BASHPID
is unset, it loses its special properties, even if it is
subsequently reset.

BASH_ALIASES

An associative array variable whose members correspond to the internal
list of aliases as maintained by the alias builtin.
(see Bourne Shell Builtins).
Elements added to this array appear in the alias list; however,
unsetting array elements currently does not cause aliases to be removed
from the alias list.
If BASH_ALIASES
is unset, it loses its special properties, even if it is
subsequently reset.

BASH_ARGC

An array variable whose values are the number of parameters in each
frame of the current bash execution call stack. The number of
parameters to the current subroutine (shell function or script executed
with . or source) is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed onto
BASH_ARGC.
The shell sets BASH_ARGC only when in extended debugging mode
(see The Shopt Builtin
for a description of the extdebug option to the shopt
builtin).
Setting extdebug after the shell has started to execute a script,
or referencing this variable when extdebug is not set,
may result in inconsistent values.

BASH_ARGV

An array variable containing all of the parameters in the current bash
execution call stack. The final parameter of the last subroutine call
is at the top of the stack; the first parameter of the initial call is
at the bottom. When a subroutine is executed, the parameters supplied
are pushed onto BASH_ARGV.
The shell sets BASH_ARGV only when in extended debugging mode
(see The Shopt Builtin
for a description of the extdebug option to the shopt
builtin).
Setting extdebug after the shell has started to execute a script,
or referencing this variable when extdebug is not set,
may result in inconsistent values.

BASH_ARGV0

When referenced, this variable expands to the name of the shell or shell
script (identical to $0; See Special Parameters,
for the description of special parameter 0).
Assignment to BASH_ARGV0
causes the value assigned to also be assigned to $0.
If BASH_ARGV0
is unset, it loses its special properties, even if it is
subsequently reset.

BASH_CMDS

An associative array variable whose members correspond to the internal
hash table of commands as maintained by the hash builtin
(see Bourne Shell Builtins).
Elements added to this array appear in the hash table; however,
unsetting array elements currently does not cause command names to be removed
from the hash table.
If BASH_CMDS
is unset, it loses its special properties, even if it is
subsequently reset.

BASH_COMMAND

The command currently being executed or about to be executed, unless the
shell is executing a command as the result of a trap,
in which case it is the command executing at the time of the trap.

BASH_COMPAT

The value is used to set the shell’s compatibility level.
See The Shopt Builtin, for a description of the various compatibility
levels and their effects.
The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42)
corresponding to the desired compatibility level.
If BASH_COMPAT is unset or set to the empty string, the compatibility
level is set to the default for the current version.
If BASH_COMPAT is set to a value that is not one of the valid
compatibility levels, the shell prints an error message and sets the
compatibility level to the default for the current version.
The valid compatibility levels correspond to the compatibility options
accepted by the shopt builtin described above (for example,
compat42 means that 4.2 and 42 are valid values).
The current version is also a valid value.

BASH_ENV

If this variable is set when Bash is invoked to execute a shell
script, its value is expanded and used as the name of a startup file
to read before executing the script. See Bash Startup Files.

BASH_EXECUTION_STRING

The command argument to the -c invocation option.

BASH_LINENO

An array variable whose members are the line numbers in source files
where each corresponding member of FUNCNAME was invoked.
${BASH_LINENO[$i]} is the line number in the source file
(${BASH_SOURCE[$i+1]}) where
${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if
referenced within another shell function).
Use LINENO to obtain the current line number.

BASH_LOADABLES_PATH

A colon-separated list of directories in which the shell looks for
dynamically loadable builtins specified by the
enable command.

BASH_REMATCH

An array variable whose members are assigned by the ‘=~’ binary
operator to the [[ conditional command
(see Conditional Constructs).
The element with index 0 is the portion of the string
matching the entire regular expression.
The element with index n is the portion of the
string matching the nth parenthesized subexpression.
This variable is read-only.

BASH_SOURCE

An array variable whose members are the source filenames where the
corresponding shell function names in the FUNCNAME array
variable are defined.
The shell function ${FUNCNAME[$i]} is defined in the file
${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}

BASH_SUBSHELL

Incremented by one within each subshell or subshell environment when
the shell begins executing in that environment.
The initial value is 0.

BASH_VERSINFO

A readonly array variable (see Arrays)
whose members hold version information for this instance of Bash.
The values assigned to the array members are as follows:

BASH_VERSINFO[0]

The major version number (the release).

BASH_VERSINFO[1]

The minor version number (the version).

BASH_VERSINFO[2]

The patch level.

BASH_VERSINFO[3]

The build version.

BASH_VERSINFO[4]

The release status (e.g., beta1).

BASH_VERSINFO[5]

The value of MACHTYPE.

BASH_VERSION

The version number of the current instance of Bash.

BASH_XTRACEFD

If set to an integer corresponding to a valid file descriptor, Bash
will write the trace output generated when ‘set -x’
is enabled to that file descriptor.
This allows tracing output to be separated from diagnostic and error
messages.
The file descriptor is closed when BASH_XTRACEFD is unset or assigned
a new value.
Unsetting BASH_XTRACEFD or assigning it the empty string causes the
trace output to be sent to the standard error.
Note that setting BASH_XTRACEFD to 2 (the standard error file
descriptor) and then unsetting it will result in the standard error
being closed.

CHILD_MAX

Set the number of exited child status values for the shell to remember.
Bash will not allow this value to be decreased below a POSIX-mandated
minimum, and there is a maximum value (currently 8192) that this may
not exceed.
The minimum value is system-dependent.

COLUMNS

Used by the select command to determine the terminal width
when printing selection lists.
Automatically set if the checkwinsize option is enabled
(see The Shopt Builtin), or in an interactive shell upon receipt of a
SIGWINCH.

COMP_CWORD

An index into ${COMP_WORDS} of the word containing the current
cursor position.
This variable is available only in shell functions invoked by the
programmable completion facilities (see Programmable Completion).

COMP_LINE

The current command line.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see Programmable Completion).

COMP_POINT

The index of the current cursor position relative to the beginning of
the current command.
If the current cursor position is at the end of the current command,
the value of this variable is equal to ${#COMP_LINE}.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see Programmable Completion).

COMP_TYPE

Set to an integer value corresponding to the type of completion attempted
that caused a completion function to be called:
TAB, for normal completion,
‘?’, for listing completions after successive tabs,
‘!’, for listing alternatives on partial word completion,
‘@’, to list completions if the word is not unmodified,
or
‘%’, for menu completion.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see Programmable Completion).

COMP_KEY

The key (or final key of a key sequence) used to invoke the current
completion function.

COMP_WORDBREAKS

The set of characters that the Readline library treats as word
separators when performing word completion.
If COMP_WORDBREAKS is unset, it loses its special properties,
even if it is subsequently reset.

COMP_WORDS

An array variable consisting of the individual
words in the current command line.
The line is split into words as Readline would split it, using
COMP_WORDBREAKS as described above.
This variable is available only in shell functions invoked by the
programmable completion facilities (see Programmable Completion).

COMPREPLY

An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable completion
facility (see Programmable Completion).
Each array element contains one possible completion.

COPROC

An array variable created to hold the file descriptors
for output from and input to an unnamed coprocess (see Coprocesses).

DIRSTACK

An array variable containing the current contents of the directory stack.
Directories appear in the stack in the order they are displayed by the
dirs builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the pushd and popd
builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory.
If DIRSTACK is unset, it loses its special properties, even if
it is subsequently reset.

EMACS

If Bash finds this variable in the environment when the shell
starts with value ‘t’, it assumes that the shell is running in an
Emacs shell buffer and disables line editing.

ENV

Similar to BASH_ENV; used when the shell is invoked in
POSIX Mode (see Bash POSIX Mode).

EPOCHREALTIME

Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch as a floating point value with micro-second granularity
(see the documentation for the C library function time for the
definition of Epoch).
Assignments to EPOCHREALTIME are ignored.
If EPOCHREALTIME is unset, it loses its special properties, even if
it is subsequently reset.

EPOCHSECONDS

Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see the documentation for the C library function
time for the definition of Epoch).
Assignments to EPOCHSECONDS are ignored.
If EPOCHSECONDS is unset, it loses its special properties, even if
it is subsequently reset.

EUID

The numeric effective user id of the current user. This variable
is readonly.

EXECIGNORE

A colon-separated list of shell patterns (see Pattern Matching)
defining the list of filenames to be ignored by command search using
PATH.
Files whose full pathnames match one of these patterns are not considered
executable files for the purposes of completion and command execution
via PATH lookup.
This does not affect the behavior of the [, test, and [[
commands.
Full pathnames in the command hash table are not subject to EXECIGNORE.
Use this variable to ignore shared library files that have the executable
bit set, but are not executable files.
The pattern matching honors the setting of the extglob shell
option.

FCEDIT

The editor used as a default by the -e option to the fc
builtin command.

FIGNORE

A colon-separated list of suffixes to ignore when performing
filename completion.
A filename whose suffix matches one of the entries in
FIGNORE
is excluded from the list of matched filenames. A sample
value is ‘.o:~’

FUNCNAME

An array variable containing the names of all shell functions
currently in the execution call stack.
The element with index 0 is the name of any currently-executing
shell function.
The bottom-most element (the one with the highest index)
is "main".
This variable exists only when a shell function is executing.
Assignments to FUNCNAME have no effect.
If FUNCNAME is unset, it loses its special properties, even if
it is subsequently reset.

This variable can be used with BASH_LINENO and BASH_SOURCE.
Each element of FUNCNAME has corresponding elements in
BASH_LINENO and BASH_SOURCE to describe the call stack.
For instance, ${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}.
The caller builtin displays the current call stack using this
information.

FUNCNEST

If set to a numeric value greater than 0, defines a maximum function
nesting level. Function invocations that exceed this nesting level
will cause the current command to abort.

GLOBIGNORE

A colon-separated list of patterns defining the set of file names to
be ignored by filename expansion.
If a file name matched by a filename expansion pattern also matches one
of the patterns in GLOBIGNORE, it is removed from the list
of matches.
The pattern matching honors the setting of the extglob shell
option.

GROUPS

An array variable containing the list of groups of which the current
user is a member.
Assignments to GROUPS have no effect.
If GROUPS is unset, it loses its special properties, even if it is
subsequently reset.

histchars

Up to three characters which control history expansion, quick
substitution, and tokenization (see History Interaction).
The first character is the
history expansion character, that is, the character which signifies the
start of a history expansion, normally ‘!’. The second character is the
character which signifies ‘quick substitution’ when seen as the first
character on a line, normally ‘^’. The optional third character is the
character which indicates that the remainder of the line is a comment when
found as the first character of a word, usually ‘#’. The history
comment character causes history substitution to be skipped for the
remaining words on the line. It does not necessarily cause the shell
parser to treat the rest of the line as a comment.

HISTCMD

The history number, or index in the history list, of the current
command. If HISTCMD is unset, it loses its special properties,
even if it is subsequently reset.

HISTCONTROL

A colon-separated list of values controlling how commands are saved on
the history list.
If the list of values includes ‘ignorespace’, lines which begin
with a space character are not saved in the history list.
A value of ‘ignoredups’ causes lines which match the previous
history entry to not be saved.
A value of ‘ignoreboth’ is shorthand for
‘ignorespace’ and ‘ignoredups’.
A value of ‘erasedups’ causes all previous lines matching the
current line to be removed from the history list before that line
is saved.
Any value not in the above list is ignored.
If HISTCONTROL is unset, or does not include a valid value,
all lines read by the shell parser are saved on the history list,
subject to the value of HISTIGNORE.
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
HISTCONTROL.

HISTFILE

The name of the file to which the command history is saved. The
default value is ~/.bash_history.

HISTFILESIZE

The maximum number of lines contained in the history file.
When this variable is assigned a value, the history file is truncated,
if necessary, to contain no more than that number of lines
by removing the oldest entries.
The history file is also truncated to this size after
writing it when a shell exits.
If the value is 0, the history file is truncated to zero size.
Non-numeric values and numeric values less than zero inhibit truncation.
The shell sets the default value to the value of HISTSIZE
after reading any startup files.

HISTIGNORE

A colon-separated list of patterns used to decide which command
lines should be saved on the history list. Each pattern is
anchored at the beginning of the line and must match the complete
line (no implicit ‘*’ is appended). Each pattern is tested
against the line after the checks specified by HISTCONTROL
are applied. In addition to the normal shell pattern matching
characters, ‘&’ matches the previous history line. ‘&’
may be escaped using a backslash; the backslash is removed
before attempting a match.
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
HISTIGNORE.
The pattern matching honors the setting of the extglob shell
option.

HISTIGNORE subsumes the function of HISTCONTROL. A
pattern of ‘&’ is identical to ignoredups, and a
pattern of ‘[ ]*’ is identical to ignorespace.
Combining these two patterns, separating them with a colon,
provides the functionality of ignoreboth.

HISTSIZE

The maximum number of commands to remember on the history list.
If the value is 0, commands are not saved in the history list.
Numeric values less than zero result in every command being saved
on the history list (there is no limit).
The shell sets the default value to 500 after reading any startup files.

HISTTIMEFORMAT

If this variable is set and not null, its value is used as a format string
for strftime to print the time stamp associated with each history
entry displayed by the history builtin.
If this variable is set, time stamps are written to the history file so
they may be preserved across shell sessions.
This uses the history comment character to distinguish timestamps from
other history lines.

HOSTFILE

Contains the name of a file in the same format as /etc/hosts that
should be read when the shell needs to complete a hostname.
The list of possible hostname completions may be changed while the shell
is running;
the next time hostname completion is attempted after the
value is changed, Bash adds the contents of the new file to the
existing list.
If HOSTFILE is set, but has no value, or does not name a readable file,
Bash attempts to read
/etc/hosts to obtain the list of possible hostname completions.
When HOSTFILE is unset, the hostname list is cleared.

HOSTNAME

The name of the current host.

HOSTTYPE

A string describing the machine Bash is running on.

IGNOREEOF

Controls the action of the shell on receipt of an EOF character
as the sole input. If set, the value denotes the number
of consecutive EOF characters that can be read as the
first character on an input line
before the shell will exit. If the variable exists but does not
have a numeric value, or has no value, then the default is 10.
If the variable does not exist, then EOF signifies the end of
input to the shell. This is only in effect for interactive shells.

INPUTRC

The name of the Readline initialization file, overriding the default
of ~/.inputrc.

INSIDE_EMACS

If Bash finds this variable in the environment when the shell
starts, it assumes that the shell is running in an Emacs shell buffer
and may disable line editing depending on the value of TERM.

LANG

Used to determine the locale category for any category not specifically
selected with a variable starting with LC_.

LC_ALL

This variable overrides the value of LANG and any other
LC_ variable specifying a locale category.

LC_COLLATE

This variable determines the collation order used when sorting the
results of filename expansion, and
determines the behavior of range expressions, equivalence classes,
and collating sequences within filename expansion and pattern matching
(see Filename Expansion).

LC_CTYPE

This variable determines the interpretation of characters and the
behavior of character classes within filename expansion and pattern
matching (see Filename Expansion).

LC_MESSAGES

This variable determines the locale used to translate double-quoted
strings preceded by a ‘$’ (see Locale Translation).

LC_NUMERIC

This variable determines the locale category used for number formatting.

LC_TIME

This variable determines the locale category used for data and time
formatting.

LINENO

The line number in the script or shell function currently executing.

LINES

Used by the select command to determine the column length
for printing selection lists.
Automatically set if the checkwinsize option is enabled
(see The Shopt Builtin), or in an interactive shell upon receipt of a
SIGWINCH.

MACHTYPE

A string that fully describes the system type on which Bash
is executing, in the standard GNUcpu-company-system format.

MAILCHECK

How often (in seconds) that the shell should check for mail in the
files specified in the MAILPATH or MAIL variables.
The default is 60 seconds. When it is time to check
for mail, the shell does so before displaying the primary prompt.
If this variable is unset, or set to a value that is not a number
greater than or equal to zero, the shell disables mail checking.

MAPFILE

An array variable created to hold the text read by the
mapfile builtin when no variable name is supplied.

OLDPWD

The previous working directory as set by the cd builtin.

OPTERR

If set to the value 1, Bash displays error messages
generated by the getopts builtin command.

OSTYPE

A string describing the operating system Bash is running on.

PIPESTATUS

An array variable (see Arrays)
containing a list of exit status values from the processes
in the most-recently-executed foreground pipeline (which may
contain only a single command).

POSIXLY_CORRECT

If this variable is in the environment when Bash starts, the shell
enters POSIX mode (see Bash POSIX Mode) before reading the
startup files, as if the --posix invocation option had been supplied.
If it is set while the shell is running, Bash enables POSIX mode,
as if the command

set -o posix

had been executed.
When the shell enters POSIX mode, it sets this variable if it was
not already set.

PPID

The process ID of the shell’s parent process. This variable
is readonly.

PROMPT_COMMAND

If set, the value is interpreted as a command to execute
before the printing of each primary prompt ($PS1).

PROMPT_DIRTRIM

If set to a number greater than zero, the value is used as the number of
trailing directory components to retain when expanding the \w and
\W prompt string escapes (see Controlling the Prompt).
Characters removed are replaced with an ellipsis.

PS0

The value of this parameter is expanded like PS1
and displayed by interactive shells after reading a command
and before the command is executed.

PS3

The value of this variable is used as the prompt for the
select command. If this variable is not set, the
select command prompts with ‘#? ’

PS4

The value of this parameter is expanded like PS1
and the expanded value is the prompt printed before the command line
is echoed when the -x option is set (see The Set Builtin).
The first character of the expanded value is replicated multiple times,
as necessary, to indicate multiple levels of indirection.
The default is ‘+ ’.

PWD

The current working directory as set by the cd builtin.

RANDOM

Each time this parameter is referenced, a random integer
between 0 and 32767 is generated. Assigning a value to this
variable seeds the random number generator.

READLINE_LINE

The contents of the Readline line buffer, for use
with ‘bind -x’ (see Bash Builtins).

READLINE_POINT

The position of the insertion point in the Readline line buffer, for use
with ‘bind -x’ (see Bash Builtins).

REPLY

The default variable for the read builtin.

SECONDS

This variable expands to the number of seconds since the
shell was started. Assignment to this variable resets
the count to the value assigned, and the expanded value
becomes the value assigned plus the number of seconds
since the assignment.

SHELL

The full pathname to the shell is kept in this environment variable.
If it is not set when the shell starts,
Bash assigns to it the full pathname of the current user’s login shell.

SHELLOPTS

A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the -o option to the
set builtin command (see The Set Builtin).
The options appearing in SHELLOPTS are those reported
as ‘on’ by ‘set -o’.
If this variable is in the environment when Bash
starts up, each shell option in the list will be enabled before
reading any startup files. This variable is readonly.

SHLVL

Incremented by one each time a new instance of Bash is started. This is
intended to be a count of how deeply your Bash shells are nested.

TIMEFORMAT

The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the time
reserved word should be displayed.
The ‘%’ character introduces an
escape sequence that is expanded to a time value or other
information.
The escape sequences and their meanings are as
follows; the braces denote optional portions.

%%

A literal ‘%’.

%[p][l]R

The elapsed time in seconds.

%[p][l]U

The number of CPU seconds spent in user mode.

%[p][l]S

The number of CPU seconds spent in system mode.

%P

The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number of
fractional digits after a decimal point.
A value of 0 causes no decimal point or fraction to be output.
At most three places after the decimal point may be specified; values
of p greater than 3 are changed to 3.
If p is not specified, the value 3 is used.

The optional l specifies a longer format, including minutes, of
the form MMmSS.FFs.
The value of p determines whether or not the fraction is included.

If this variable is not set, Bash acts as if it had the value

$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'

If the value is null, no timing information is displayed.
A trailing newline is added when the format string is displayed.

TMOUT

If set to a value greater than zero, TMOUT is treated as the
default timeout for the read builtin (see Bash Builtins).
The select command (see Conditional Constructs) terminates
if input does not arrive after TMOUT seconds when input is coming
from a terminal.

In an interactive shell, the value is interpreted as
the number of seconds to wait for a line of input after issuing
the primary prompt.
Bash
terminates after waiting for that number of seconds if a complete
line of input does not arrive.

TMPDIR

If set, Bash uses its value as the name of a directory in which
Bash creates temporary files for the shell’s use.

UID

The numeric real user id of the current user. This variable is readonly.

All of the single-character options used with the set builtin
(see The Set Builtin) can be used as options when the shell is invoked.
In addition, there are several multi-character
options that you can use. These options must appear on the command
line before the single-character options to be recognized.

--debugger

Arrange for the debugger profile to be executed before the shell
starts. Turns on extended debugging mode (see The Shopt Builtin
for a description of the extdebug option to the shopt
builtin).

--dump-po-strings

A list of all double-quoted strings preceded by ‘$’
is printed on the standard output
in the GNUgettext PO (portable object) file format.
Equivalent to -D except for the output format.

--dump-strings

Equivalent to -D.

--help

Display a usage message on standard output and exit successfully.

--init-file filename

--rcfile filename

Execute commands from filename (instead of ~/.bashrc)
in an interactive shell.

--login

Equivalent to -l.

--noediting

Do not use the GNU Readline library (see Command Line Editing)
to read command lines when the shell is interactive.

--noprofile

Don’t load the system-wide startup file /etc/profile
or any of the personal initialization files
~/.bash_profile, ~/.bash_login, or ~/.profile
when Bash is invoked as a login shell.

--norc

Don’t read the ~/.bashrc initialization file in an
interactive shell. This is on by default if the shell is
invoked as sh.

--posix

Change the behavior of Bash where the default operation differs
from the POSIX standard to match the standard. This
is intended to make Bash behave as a strict superset of that
standard. See Bash POSIX Mode, for a description of the Bash
POSIX mode.

Show version information for this instance of
Bash on the standard output and exit successfully.

There are several single-character options that may be supplied at
invocation which are not available with the set builtin.

-c

Read and execute commands from the first non-option argument
command_string, then exit.
If there are arguments after the command_string,
the first argument is assigned to $0
and any remaining arguments are assigned to the positional parameters.
The assignment to $0 sets the name of the shell, which is used
in warning and error messages.

-i

Force the shell to run interactively. Interactive shells are
described in Interactive Shells.

-l

Make this shell act as if it had been directly invoked by login.
When the shell is interactive, this is equivalent to starting a
login shell with ‘exec -l bash’.
When the shell is not interactive, the login shell startup files will
be executed.
‘exec bash -l’ or ‘exec bash --login’
will replace the current shell with a Bash login shell.
See Bash Startup Files, for a description of the special behavior
of a login shell.

If this option is present, or if no arguments remain after option
processing, then commands are read from the standard input.
This option allows the positional parameters to be set
when invoking an interactive shell or when reading input
through a pipe.

-D

A list of all double-quoted strings preceded by ‘$’
is printed on the standard output.
These are the strings that
are subject to language translation when the current locale
is not C or POSIX (see Locale Translation).
This implies the -n option; no commands will be executed.

[-+]O [shopt_option]

shopt_option is one of the shell options accepted by the
shopt builtin (see The Shopt Builtin).
If shopt_option is present, -O sets the value of that option;
+O unsets it.
If shopt_option is not supplied, the names and values of the shell
options accepted by shopt are printed on the standard output.
If the invocation option is +O, the output is displayed in a format
that may be reused as input.

--

A -- signals the end of options and disables further option
processing.
Any arguments after the -- are treated as filenames and arguments.

A login shell is one whose first character of argument zero is
‘-’, or one invoked with the --login option.

An interactive shell is one started without non-option arguments,
unless -s is specified,
without specifying the -c option, and whose input and output are both
connected to terminals (as determined by isatty(3)), or one
started with the -i option. See Interactive Shells, for more
information.

If arguments remain after option processing, and neither the
-c nor the -s
option has been supplied, the first argument is assumed to
be the name of a file containing shell commands (see Shell Scripts).
When Bash is invoked in this fashion, $0
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
Bash reads and executes commands from this file, then exits.
Bash’s exit status is the exit status of the last command executed
in the script. If no commands are executed, the exit status is 0.

6.2 Bash Startup Files

This section describes how Bash executes its startup files.
If any of the files exist but cannot be read, Bash reports an error.
Tildes are expanded in filenames as described above under
Tilde Expansion (see Tilde Expansion).

Invoked as an interactive login shell, or with --login

When Bash is invoked as an interactive login shell, or as a
non-interactive shell with the --login option, it first reads and
executes commands from the file /etc/profile, if that file exists.
After reading that file, it looks for ~/.bash_profile,
~/.bash_login, and ~/.profile, in that order, and reads
and executes commands from the first one that exists and is readable.
The --noprofile option may be used when the shell is started to
inhibit this behavior.

When an interactive login shell exits,
or a non-interactive login shell executes the exit builtin command,
Bash reads and executes commands from
the file ~/.bash_logout, if it exists.

Invoked as an interactive non-login shell

When an interactive shell that is not a login shell is started, Bash
reads and executes commands from ~/.bashrc, if that file exists.
This may be inhibited by using the --norc option.
The --rcfile file option will force Bash to read and
execute commands from file instead of ~/.bashrc.

So, typically, your ~/.bash_profile contains the line

if [ -f ~/.bashrc ]; then . ~/.bashrc; fi

after (or before) any login-specific initializations.

Invoked non-interactively

When Bash is started non-interactively, to run a shell script,
for example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the
following command were executed:

if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but the value of the PATH variable is not used to search for the
filename.

As noted above, if a non-interactive shell is invoked with the
--login option, Bash attempts to read and execute commands from the
login shell startup files.

Invoked with name sh

If Bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely as
possible, while conforming to the POSIX standard as well.

When invoked as an interactive login shell, or as a non-interactive
shell with the --login option, it first attempts to read
and execute commands from /etc/profile and ~/.profile, in
that order.
The --noprofile option may be used to inhibit this behavior.
When invoked as an interactive shell with the name sh, Bash
looks for the variable ENV, expands its value if it is defined,
and uses the expanded value as the name of a file to read and execute.
Since a shell invoked as sh does not attempt to read and execute
commands from any other startup files, the --rcfile option has
no effect.
A non-interactive shell invoked with the name sh does not attempt
to read any other startup files.

When invoked as sh, Bash enters POSIX mode after
the startup files are read.

Invoked in POSIX mode

When Bash is started in POSIX mode, as with the
--posix command line option, it follows the POSIX standard
for startup files.
In this mode, interactive shells expand the ENV variable
and commands are read and executed from the file whose name is the
expanded value.
No other startup files are read.

Invoked by remote shell daemon

Bash attempts to determine when it is being run with its standard input
connected to a network connection, as when executed by the remote shell
daemon, usually rshd, or the secure shell daemon sshd.
If Bash determines it is being run in
this fashion, it reads and executes commands from ~/.bashrc, if that
file exists and is readable.
It will not do this if invoked as sh.
The --norc option may be used to inhibit this behavior, and the
--rcfile option may be used to force another file to be read, but
neither rshd nor sshd generally invoke the shell with those
options or allow them to be specified.

Invoked with unequal effective and real UID/GIDs

If Bash is started with the effective user (group) id not equal to the
real user (group) id, and the -p option is not supplied, no startup
files are read, shell functions are not inherited from the environment,
the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE
variables, if they appear in the environment, are ignored, and the effective
user id is set to the real user id.
If the -p option is supplied at invocation, the startup behavior is
the same, but the effective user id is not reset.

6.3.1 What is an Interactive Shell?

An interactive shell
is one started without non-option arguments, unless -s is
specified, without specifying the -c option, and
whose input and error output are both
connected to terminals (as determined by isatty(3)),
or one started with the -i option.

An interactive shell generally reads from and writes to a user’s
terminal.

The -s invocation option may be used to set the positional parameters
when an interactive shell is started.

6.3.3 Interactive Shell Behavior

Job Control (see Job Control) is enabled by default. When job
control is in effect, Bash ignores the keyboard-generated job control
signals SIGTTIN, SIGTTOU, and SIGTSTP.

Bash expands and displays PS1 before reading the first line
of a command, and expands and displays PS2 before reading the
second and subsequent lines of a multi-line command.
Bash expands and displays PS0 after it reads a command but before
executing it.
See Controlling the Prompt, for a complete list of prompt
string escape sequences.

Bash executes the value of the PROMPT_COMMAND variable as a command
before printing the primary prompt, $PS1
(see Bash Variables).

Bash inspects the value of the ignoreeof option to set -o
instead of exiting immediately when it receives an EOF on its
standard input when reading a command (see The Set Builtin).

Command history (see Bash History Facilities)
and history expansion (see History Interaction)
are enabled by default.
Bash will save the command history to the file named by $HISTFILE
when a shell with history enabled exits.

6.4 Bash Conditional Expressions

Conditional expressions are used by the [[ compound command
and the test and [ builtin commands. The test
and [ commands determine their behavior based on the number
of arguments; see the descriptions of those commands for any other
command-specific actions.

Expressions may be unary or binary,
and are formed from the following primaries.
Unary expressions are often used to examine the status of a file.
There are string operators and numeric comparison operators as well.
Bash handles several filenames specially when they are used in
expressions.
If the operating system on which Bash is running provides these
special files, Bash will use them; otherwise it will emulate them
internally with this behavior:
If the file argument to one of the primaries is of the form
/dev/fd/N, then file descriptor N is checked.
If the file argument to one of the primaries is one of
/dev/stdin, /dev/stdout, or /dev/stderr, file
descriptor 0, 1, or 2, respectively, is checked.

When used with [[, the ‘<’ and ‘>’ operators sort
lexicographically using the current locale.
The test command uses ASCII ordering.

Unless otherwise specified, primaries that operate on files follow symbolic
links and operate on the target of the link, rather than the link itself.

-a file

True if file exists.

-b file

True if file exists and is a block special file.

-c file

True if file exists and is a character special file.

-d file

True if file exists and is a directory.

-e file

True if file exists.

-f file

True if file exists and is a regular file.

-g file

True if file exists and its set-group-id bit is set.

-h file

True if file exists and is a symbolic link.

-k file

True if file exists and its "sticky" bit is set.

-p file

True if file exists and is a named pipe (FIFO).

-r file

True if file exists and is readable.

-s file

True if file exists and has a size greater than zero.

-t fd

True if file descriptor fd is open and refers to a terminal.

-u file

True if file exists and its set-user-id bit is set.

-w file

True if file exists and is writable.

-x file

True if file exists and is executable.

-G file

True if file exists and is owned by the effective group id.

-L file

True if file exists and is a symbolic link.

-N file

True if file exists and has been modified since it was last read.

-O file

True if file exists and is owned by the effective user id.

-S file

True if file exists and is a socket.

file1 -ef file2

True if file1 and file2 refer to the same device and
inode numbers.

file1 -nt file2

True if file1 is newer (according to modification date)
than file2, or if file1 exists and file2 does not.

file1 -ot file2

True if file1 is older than file2,
or if file2 exists and file1 does not.

-o optname

True if the shell option optname is enabled.
The list of options appears in the description of the -o
option to the set builtin (see The Set Builtin).

-v varname

True if the shell variable varname is set (has been assigned a value).

-R varname

True if the shell variable varname is set and is a name reference.

-z string

True if the length of string is zero.

-n string

string

True if the length of string is non-zero.

string1 == string2

string1 = string2

True if the strings are equal.
When used with the [[ command, this performs pattern matching as
described above (see Conditional Constructs).

‘=’ should be used with the test command for POSIX conformance.

string1 != string2

True if the strings are not equal.

string1 < string2

True if string1 sorts before string2 lexicographically.

string1 > string2

True if string1 sorts after string2 lexicographically.

arg1 OP arg2

OP is one of
‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’.
These arithmetic binary operators return true if arg1
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to arg2,
respectively. Arg1 and arg2
may be positive or negative integers.
When used with the [[ command, Arg1 and Arg2
are evaluated as arithmetic expressions (see Shell Arithmetic).

6.5 Shell Arithmetic

The shell allows arithmetic expressions to be evaluated, as one of
the shell expansions or by using the (( compound command, the
let builtin, or the -i option to the declare builtin.

Evaluation is done in fixed-width integers with no check for overflow,
though division by 0 is trapped and flagged as an error.
The operators and their precedence, associativity, and values
are the same as in the C language.
The following list of operators is grouped into levels of
equal-precedence operators.
The levels are listed in order of decreasing precedence.

id++ id--

variable post-increment and post-decrement

++id --id

variable pre-increment and pre-decrement

- +

unary minus and plus

! ~

logical and bitwise negation

**

exponentiation

* / %

multiplication, division, remainder

+ -

addition, subtraction

<< >>

left and right bitwise shifts

<= >= < >

comparison

== !=

equality and inequality

&

bitwise AND

^

bitwise exclusive OR

|

bitwise OR

&&

logical AND

||

logical OR

expr ? expr : expr

conditional operator

= *= /= %= += -= <<= >>= &= ^= |=

assignment

expr1 , expr2

comma

Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated.
Within an expression, shell variables may also be referenced by name
without using the parameter expansion syntax.
A shell variable that is null or unset evaluates to 0 when referenced
by name without using the parameter expansion syntax.
The value of a variable is evaluated as an arithmetic expression
when it is referenced, or when a variable which has been given the
integer attribute using ‘declare -i’ is assigned a value.
A null value evaluates to 0.
A shell variable need not have its integer attribute turned on
to be used in an expression.

Constants with a leading 0 are interpreted as octal numbers.
A leading ‘0x’ or ‘0X’ denotes hexadecimal. Otherwise,
numbers take the form [base#]n, where the optional base
is a decimal number between 2 and 64 representing the arithmetic
base, and n is a number in that base.
If base# is omitted, then base 10 is used.
When specifying n,
the digits greater than 9 are represented by the lowercase letters,
the uppercase letters, ‘@’, and ‘_’, in that order.
If base is less than or equal to 36, lowercase and uppercase
letters may be used interchangeably to represent numbers between 10
and 35.

Operators are evaluated in order of precedence. Sub-expressions in
parentheses are evaluated first and may override the precedence
rules above.

6.6 Aliases

Aliases allow a string to be substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of aliases that may be set and unset with
the alias and unalias builtin commands.

The first word of each simple command, if unquoted, is checked to see
if it has an alias.
If so, that word is replaced by the text of the alias.
The characters ‘/’, ‘$’, ‘`’, ‘=’ and any of the
shell metacharacters or quoting characters listed above may not appear
in an alias name.
The replacement text may contain any valid
shell input, including shell metacharacters.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time.
This means that one may alias ls to "ls -F",
for instance, and Bash does not try to recursively expand the
replacement text.
If the last character of the alias value is a
blank, then the next command word following the
alias is also checked for alias expansion.

Aliases are created and listed with the alias
command, and removed with the unalias command.

There is no mechanism for using arguments in the replacement text,
as in csh.
If arguments are needed, a shell function should be used
(see Shell Functions).

Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see The Shopt Builtin).

The rules concerning the definition and use of aliases are
somewhat confusing. Bash
always reads at least one complete line of input,
and all lines that make up a compound command,
before executing any of the commands on that line or the compound command.
Aliases are expanded when a
command is read, not when it is executed. Therefore, an
alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a command. As a consequence, aliases
defined in a function are not available until after that
function is executed. To be safe, always put
alias definitions on a separate line, and do not use alias
in compound commands.

6.7 Arrays

Bash provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array;
the declare builtin will explicitly declare an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously.
Indexed arrays are referenced using integers (including arithmetic
expressions (see Shell Arithmetic)) and are zero-based;
associative arrays use arbitrary strings.
Unless otherwise noted, indexed array indices must be non-negative integers.

An indexed array is created automatically if any variable is assigned to
using the syntax

name[subscript]=value

The subscript
is treated as an arithmetic expression that must evaluate to a number.
To explicitly declare an array, use

declare -a name

The syntax

declare -a name[subscript]

is also accepted; the subscript is ignored.

Associative arrays are created using

declare -A name

Attributes may be
specified for an array variable using the declare and
readonly builtins. Each attribute applies to all members of
an array.

Arrays are assigned to using compound assignments of the form

name=(value1value2 … )

where each
value is of the form [subscript]=string.
Indexed array assignments do not require anything but string.
When assigning to indexed arrays, if
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one. Indexing starts at zero.

When assigning to an associative array, the subscript is required.

This syntax is also accepted by the declare
builtin. Individual array elements may be assigned to using the
name[subscript]=value syntax introduced above.

When assigning to an indexed array, if name
is subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
name, so negative indices count back from the end of the
array, and an index of -1 references the last element.

Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid
conflicts with the shell’s filename expansion operators. If the
subscript is ‘@’ or ‘*’, the word expands to all members
of the array name. These subscripts differ only when the word
appears within double quotes.
If the word is double-quoted,
${name[*]} expands to a single word with
the value of each array member separated by the first character of the
IFS variable, and ${name[@]} expands each element of
name to a separate word. When there are no array members,
${name[@]} expands to nothing.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
This is analogous to the
expansion of the special parameters ‘@’ and ‘*’.
${#name[subscript]} expands to the length of
${name[subscript]}.
If subscript is ‘@’ or
‘*’, the expansion is the number of elements in the array.
If the subscript
used to reference an element of an indexed array
evaluates to a number less than zero, it is
interpreted as relative to one greater than the maximum index of the array,
so negative indices count back from the end of the array,
and an index of -1 refers to the last element.

Referencing an array variable without a subscript is equivalent to
referencing with a subscript of 0.
Any reference to a variable using a valid subscript is legal, and
bash will create an array if necessary.

An array variable is considered set if a subscript has been assigned a
value. The null string is a valid value.

It is possible to obtain the keys (indices) of an array as well as the values.
${!name[@]} and ${!name[*]} expand to the indices
assigned in array variable name.
The treatment when in double quotes is similar to the expansion of the
special parameters ‘@’ and ‘*’ within double quotes.

The unset builtin is used to destroy arrays.
unset name[subscript]
destroys the array element at index subscript.
Negative subscripts to indexed arrays are interpreted as described above.
Unsetting the last element of an array variable does not unset the variable.
unset name, where name is an array, removes the
entire array. A subscript of ‘*’ or ‘@’ also removes the
entire array.

When using a variable name with a subscript as an argument to a command,
such as with unset, without using the word expansion syntax
described above, the argument is subject to the shell’s filename expansion.
If filename expansion is not desired, the argument should be quoted.

The declare, local, and readonly
builtins each accept a -a option to specify an indexed
array and a -A option to specify an associative array.
If both options are supplied, -A takes precedence.
The read builtin accepts a -a
option to assign a list of words read from the standard input
to an array, and can read values from the standard input into
individual array elements. The set and declare
builtins display array values in a way that allows them to be
reused as input.

6.8 The Directory Stack

The directory stack is a list of recently-visited directories. The
pushd builtin adds directories to the stack as it changes
the current directory, and the popd builtin removes specified
directories from the stack and changes the current directory to
the directory removed. The dirs builtin displays the contents
of the directory stack. The current directory is always the "top"
of the directory stack.

The contents of the directory stack are also visible
as the value of the DIRSTACK shell variable.

6.8.1 Directory Stack Builtins

dirs

dirs [-clpv] [+N | -N]

Display the list of currently remembered directories. Directories
are added to the list with the pushd command; the
popd command removes directories from the list.
The current directory is always the first directory in the stack.

-c

Clears the directory stack by deleting all of the elements.

-l

Produces a listing using full pathnames;
the default listing format uses a tilde to denote the home directory.

-p

Causes dirs to print the directory stack with one entry per
line.

-v

Causes dirs to print the directory stack with one entry per
line, prefixing each entry with its index in the stack.

+N

Displays the Nth directory (counting from the left of the
list printed by dirs when invoked without options), starting
with zero.

-N

Displays the Nth directory (counting from the right of the
list printed by dirs when invoked without options), starting
with zero.

popd

popd [-n] [+N | -N]

When no arguments are given, popd
removes the top directory from the stack and
performs a cd to the new top directory.
The elements are numbered from 0 starting at the first directory
listed with dirs; that is, popd is equivalent to popd +0.

-n

Suppresses the normal change of directory when removing directories
from the stack, so that only the stack is manipulated.

+N

Removes the Nth directory (counting from the left of the
list printed by dirs), starting with zero.

-N

Removes the Nth directory (counting from the right of the
list printed by dirs), starting with zero.

pushd

pushd [-n] [+N | -N | dir]

Save the current directory on the top of the directory stack
and then cd to dir.
With no arguments, pushd exchanges the top two directories
and makes the new top the current directory.

-n

Suppresses the normal change of directory when rotating or
adding directories to the stack, so that only the stack is manipulated.

+N

Brings the Nth directory (counting from the left of the
list printed by dirs, starting with zero) to the top of
the list by rotating the stack.

-N

Brings the Nth directory (counting from the right of the
list printed by dirs, starting with zero) to the top of
the list by rotating the stack.

dir

Makes dir be the top of the stack, making
it the new current directory as if it had been supplied as an argument
to the cd builtin.

6.9 Controlling the Prompt

The value of the variable PROMPT_COMMAND is examined just before
Bash prints each primary prompt. If PROMPT_COMMAND is set and
has a non-null value, then the
value is executed just as if it had been typed on the command line.

In addition, the following table describes the special characters which
can appear in the prompt variables PS0, PS1, PS2, and
PS4:

\a

A bell character.

\d

The date, in "Weekday Month Date" format (e.g., "Tue May 26").

\D{format}

The format is passed to strftime(3) and the result is inserted
into the prompt string; an empty format results in a locale-specific
time representation. The braces are required.

\e

An escape character.

\h

The hostname, up to the first ‘.’.

\H

The hostname.

\j

The number of jobs currently managed by the shell.

\l

The basename of the shell’s terminal device name.

\n

A newline.

\r

A carriage return.

\s

The name of the shell, the basename of $0 (the portion
following the final slash).

\t

The time, in 24-hour HH:MM:SS format.

\T

The time, in 12-hour HH:MM:SS format.

\@

The time, in 12-hour am/pm format.

\A

The time, in 24-hour HH:MM format.

\u

The username of the current user.

\v

The version of Bash (e.g., 2.00)

\V

The release of Bash, version + patchlevel (e.g., 2.00.0)

\w

The current working directory, with $HOME abbreviated with a tilde
(uses the $PROMPT_DIRTRIM variable).

\W

The basename of $PWD, with $HOME abbreviated with a tilde.

\!

The history number of this command.

\#

The command number of this command.

\$

If the effective uid is 0, #, otherwise $.

\nnn

The character whose ASCII code is the octal value nnn.

\\

A backslash.

\[

Begin a sequence of non-printing characters. This could be used to
embed a terminal control sequence into the prompt.

\]

End a sequence of non-printing characters.

The command number and the history number are usually different:
the history number of a command is its position in the history
list, which may include commands restored from the history file
(see Bash History Facilities), while the command number is
the position in the sequence of commands executed during the current
shell session.

After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the
promptvars shell option (see The Shopt Builtin).
This can have unwanted side effects if escaped portions of the string
appear within command substitution or contain characters special to
word expansion.

6.10 The Restricted Shell

If Bash is started with the name rbash, or the
--restricted
or
-r
option is supplied at invocation, the shell becomes restricted.
A restricted shell is used to
set up an environment more controlled than the standard shell.
A restricted shell behaves identically to bash
with the exception that the following are disallowed or not performed:

Changing directories with the cd builtin.

Setting or unsetting the values of the SHELL, PATH,
ENV, or BASH_ENV variables.

Specifying command names containing slashes.

Specifying a filename containing a slash as an argument to the .
builtin command.

Specifying a filename containing a slash as an argument to the -p
option to the hash builtin command.

Adding or deleting builtin commands with the
-f and -d options to the enable builtin.

Using the enable builtin command to enable disabled shell builtins.

Specifying the -p option to the command builtin.

Turning off restricted mode with ‘set +r’ or ‘set +o restricted’.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed
(see Shell Scripts), rbash turns off any restrictions in
the shell spawned to execute the script.

The restricted shell mode is only one component of a useful restricted
environment. It should be accompanied by setting PATH to a value
that allows execution of only a few verified commands (commands that
allow shell escapes are particularly vulnerable), leaving the user
in a non-writable directory other than his home directory after login,
not allowing the restricted shell to execute shell scripts, and cleaning
the environment of variables that cause some commands to modify their
behavior (e.g., VISUAL or PAGER).

Modern systems provide more secure ways to implement a restricted environment,
such as jails, zones, or containers.

6.11 Bash POSIX Mode

Starting Bash with the --posix command-line option or executing
‘set -o posix’ while Bash is running will cause Bash to conform more
closely to the POSIX standard by changing the behavior to
match that specified by POSIX in areas where the Bash default differs.

When invoked as sh, Bash enters POSIX mode after reading the
startup files.

The following list is what’s changed when ‘POSIX mode’ is in effect:

Bash ensures that the POSIXLY_CORRECT variable is set.

When a command in the hash table no longer exists, Bash will re-search
$PATH to find the new location. This is also available with
‘shopt -s checkhash’.

The message printed by the job control code and builtins when a job
exits with a non-zero status is ‘Done(status)’.

The message printed by the job control code and builtins when a job
is stopped is ‘Stopped(signame)’, where signame is, for
example, SIGTSTP.

Alias expansion is always enabled, even in non-interactive shells.

Reserved words appearing in a context where reserved words are recognized
do not undergo alias expansion.

The POSIXPS1 and PS2 expansions of ‘!’ to
the history number and ‘!!’ to ‘!’ are enabled,
and parameter expansion is performed on the values of PS1 and
PS2 regardless of the setting of the promptvars option.

The POSIX startup files are executed ($ENV) rather than
the normal Bash files.

Tilde expansion is only performed on assignments preceding a command
name, rather than on all assignment statements on the line.

The default history file is ~/.sh_history (this is the
default value of $HISTFILE).

Redirection operators do not perform filename expansion on the word
in the redirection unless the shell is interactive.

Redirection operators do not perform word splitting on the word in the
redirection.

Function names must be valid shell names. That is, they may not
contain characters other than letters, digits, and underscores, and
may not start with a digit. Declaring a function with an invalid name
causes a fatal syntax error in non-interactive shells.

Function names may not be the same as one of the POSIX special
builtins.

POSIX special builtins are found before shell functions
during command lookup.

When printing shell function definitions (e.g., by type), Bash does
not print the function keyword.

Literal tildes that appear as the first character in elements of
the PATH variable are not expanded as described above
under Tilde Expansion.

The time reserved word may be used by itself as a command. When
used in this way, it displays timing statistics for the shell and its
completed children. The TIMEFORMAT variable controls the format
of the timing information.

When parsing and expanding a ${…} expansion that appears within
double quotes, single quotes are no longer special and cannot be used to
quote a closing brace or other special character, unless the operator is
one of those defined to perform pattern removal. In this case, they do
not have to appear as matched pairs.

The parser does not recognize time as a reserved word if the next
token begins with a ‘-’.

The ‘!’ character does not introduce history expansion within a
double-quoted string, even if the histexpand option is enabled.

If a POSIX special builtin returns an error status, a
non-interactive shell exits. The fatal errors are those listed in
the POSIX standard, and include things like passing incorrect options,
redirection errors, variable assignment errors for assignments preceding
the command name, and so on.

A non-interactive shell exits with an error status if a variable
assignment error occurs when no command name follows the assignment
statements.
A variable assignment error occurs, for example, when trying to assign
a value to a readonly variable.

A non-interactive shell exits with an error status if a variable
assignment error occurs in an assignment statement preceding a special
builtin, but not with any other simple command.

A non-interactive shell exits with an error status if the iteration
variable in a for statement or the selection variable in a
select statement is a readonly variable.

Non-interactive shells exit if filename in .filename
is not found.

Non-interactive shells exit if a syntax error in an arithmetic expansion
results in an invalid expression.

Non-interactive shells exit if a parameter expansion error occurs.

Non-interactive shells exit if there is a syntax error in a script read
with the . or source builtins, or in a string processed by
the eval builtin.

Process substitution is not available.

While variable indirection is available, it may not be applied to the
‘#’ and ‘?’ special parameters.

When expanding the ‘*’ special parameter in a pattern context where the
expansion is double-quoted does not treat the $* as if it were
double-quoted.

Assignment statements preceding POSIX special builtins
persist in the shell environment after the builtin completes.

Assignment statements preceding shell function calls persist in the
shell environment after the function returns, as if a POSIX
special builtin command had been executed.

The command builtin does not prevent builtins that take assignment
statements as arguments from expanding them as assignment statements;
when not in POSIX mode, assignment builtins lose their assignment
statement expansion properties when preceded by command.

The bg builtin uses the required format to describe each job placed
in the background, which does not include an indication of whether the job
is the current or previous job.

The output of ‘kill -l’ prints all the signal names on a single line,
separated by spaces, without the ‘SIG’ prefix.

The kill builtin does not accept signal names with a ‘SIG’
prefix.

The export and readonly builtin commands display their
output in the format required by POSIX.

The trap builtin displays signal names without the leading
SIG.

The trap builtin doesn’t check the first argument for a possible
signal specification and revert the signal handling to the original
disposition if it is, unless that argument consists solely of digits and
is a valid signal number. If users want to reset the handler for a given
signal to the original disposition, they should use ‘-’ as the
first argument.

The . and source builtins do not search the current directory
for the filename argument if it is not found by searching PATH.

Enabling POSIX mode has the effect of setting the
inherit_errexit option, so
subshells spawned to execute command substitutions inherit the value of
the -e option from the parent shell.
When the inherit_errexit option is not enabled,
Bash clears the -e option in such subshells.

Enabling POSIX mode has the effect of setting the
shift_verbose option, so numeric arguments to shift
that exceed the number of positional parameters will result in an
error message.

When the alias builtin displays alias definitions, it does not
display them with a leading ‘alias ’ unless the -p option
is supplied.

When the set builtin is invoked without options, it does not display
shell function names and definitions.

When the set builtin is invoked without options, it displays
variable values without quotes, unless they contain shell metacharacters,
even if the result contains nonprinting characters.

When the cd builtin is invoked in logical mode, and the pathname
constructed from $PWD and the directory name supplied as an argument
does not refer to an existing directory, cd will fail instead of
falling back to physical mode.

When the cd builtin cannot change a directory because the
length of the pathname
constructed from $PWD and the directory name supplied as an argument
exceeds PATH_MAX when all symbolic links are expanded, cd will
fail instead of attempting to use only the supplied directory name.

The pwd builtin verifies that the value it prints is the same as the
current directory, even if it is not asked to check the file system with the
-P option.

When listing the history, the fc builtin does not include an
indication of whether or not a history entry has been modified.

The default editor used by fc is ed.

The type and command builtins will not report a non-executable
file as having been found, though the shell will attempt to execute such a
file if it is the only so-named file found in $PATH.

The vi editing mode will invoke the vi editor directly when
the ‘v’ command is run, instead of checking $VISUAL and
$EDITOR.

When the xpg_echo option is enabled, Bash does not attempt to interpret
any arguments to echo as options. Each argument is displayed, after
escape characters are converted.

The ulimit builtin uses a block size of 512 bytes for the -c
and -f options.

The arrival of SIGCHLD when a trap is set on SIGCHLD does
not interrupt the wait builtin and cause it to return immediately.
The trap command is run once for each child that exits.

The read builtin may be interrupted by a signal for which a trap
has been set.
If Bash receives a trapped signal while executing read, the trap
handler executes and read returns an exit status greater than 128.

Bash removes an exited background process’s status from the list of such
statuses after the wait builtin is used to obtain it.

There is other POSIX behavior that Bash does not implement by
default even when in POSIX mode.
Specifically:

The fc builtin checks $EDITOR as a program to edit history
entries if FCEDIT is unset, rather than defaulting directly to
ed. fc uses ed if EDITOR is unset.

As noted above, Bash requires the xpg_echo option to be enabled for
the echo builtin to be fully conformant.

Bash can be configured to be POSIX-conformant by default, by specifying
the --enable-strict-posix-default to configure when building
(see Optional Features).

7.1 Job Control Basics

Job control
refers to the ability to selectively stop (suspend)
the execution of processes and continue (resume)
their execution at a later point. A user typically employs
this facility via an interactive interface supplied jointly
by the operating system kernel’s terminal driver and Bash.

The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed with the
jobs command. When Bash starts a job
asynchronously, it prints a line that looks
like:

[1] 25647

indicating that this job is job number 1 and that the process ID
of the last process in the pipeline associated with this job is
25647. All of the processes in a single pipeline are members of
the same job. Bash uses the job abstraction as the
basis for job control.

To facilitate the implementation of the user interface to job
control, the operating system maintains the notion of a current terminal
process group ID. Members of this process group (processes whose
process group ID is equal to the current terminal process group
ID) receive keyboard-generated signals such as SIGINT.
These processes are said to be in the foreground. Background
processes are those whose process group ID differs from the
terminal’s; such processes are immune to keyboard-generated
signals. Only foreground processes are allowed to read from or, if
the user so specifies with stty tostop, write to the terminal.
Background processes which attempt to
read from (write to when stty tostop is in effect) the
terminal are sent a SIGTTIN (SIGTTOU)
signal by the kernel’s terminal driver,
which, unless caught, suspends the process.

If the operating system on which Bash is running supports
job control, Bash contains facilities to use it. Typing the
suspend character (typically ‘^Z’, Control-Z) while a
process is running causes that process to be stopped and returns
control to Bash. Typing the delayed suspend character
(typically ‘^Y’, Control-Y) causes the process to be stopped
when it attempts to read input from the terminal, and control to
be returned to Bash. The user then manipulates the state of
this job, using the bg command to continue it in the
background, the fg command to continue it in the
foreground, or the kill command to kill it. A ‘^Z’
takes effect immediately, and has the additional side effect of
causing pending output and typeahead to be discarded.

There are a number of ways to refer to a job in the shell. The
character ‘%’ introduces a job specification (jobspec).

Job number n may be referred to as ‘%n’.
The symbols ‘%%’ and ‘%+’ refer to the shell’s notion of the
current job, which is the last job stopped while it was in the foreground
or started in the background.
A single ‘%’ (with no accompanying job specification) also refers
to the current job.
The previous job may be referenced using ‘%-’.
If there is only a single job, ‘%+’ and ‘%-’ can both be used
to refer to that job.
In output pertaining to jobs (e.g., the output of the jobs
command), the current job is always flagged with a ‘+’, and the
previous job with a ‘-’.

A job may also be referred to
using a prefix of the name used to start it, or using a substring
that appears in its command line. For example, ‘%ce’ refers
to a stopped ce job. Using ‘%?ce’, on the
other hand, refers to any job containing the string ‘ce’ in
its command line. If the prefix or substring matches more than one job,
Bash reports an error.

Simply naming a job can be used to bring it into the foreground:
‘%1’ is a synonym for ‘fg %1’, bringing job 1 from the
background into the foreground. Similarly, ‘%1 &’ resumes
job 1 in the background, equivalent to ‘bg %1’

The shell learns immediately whenever a job changes state.
Normally, Bash waits until it is about to print a prompt
before reporting changes in a job’s status so as to not interrupt
any other output.
If the -b option to the set builtin is enabled,
Bash reports such changes immediately (see The Set Builtin).
Any trap on SIGCHLD is executed for each child process
that exits.

If an attempt to exit Bash is made while jobs are stopped, (or running, if
the checkjobs option is enabled – see The Shopt Builtin), the
shell prints a warning message, and if the checkjobs option is
enabled, lists the jobs and their statuses.
The jobs command may then be used to inspect their status.
If a second attempt to exit is made without an intervening command,
Bash does not print another warning, and any stopped jobs are terminated.

When the shell is waiting for a job or process using the wait
builtin, and job control is enabled, wait will return when the
job changes state. The -f option causes wait to wait
until the job or process terminates before returning.

7.2 Job Control Builtins

bg

bg [jobspec …]

Resume each suspended job jobspec in the background, as if it
had been started with ‘&’.
If jobspec is not supplied, the current job is used.
The return status is zero unless it is run when job control is not
enabled, or, when run with job control enabled, any
jobspec was not found or specifies a job
that was started without job control.

fg

fg [jobspec]

Resume the job jobspec in the foreground and make it the current job.
If jobspec is not supplied, the current job is used.
The return status is that of the command placed into the foreground,
or non-zero if run when job control is disabled or, when run with
job control enabled, jobspec does not specify a valid job or
jobspec specifies a job that was started without job control.

jobs

jobs [-lnprs] [jobspec]
jobs -x command [arguments]

The first form lists the active jobs. The options have the
following meanings:

-l

List process IDs in addition to the normal information.

-n

Display information only about jobs that have changed status since
the user was last notified of their status.

-p

List only the process ID of the job’s process group leader.

-r

Display only running jobs.

-s

Display only stopped jobs.

If jobspec is given,
output is restricted to information about that job.
If jobspec is not supplied, the status of all jobs is
listed.

If the -x option is supplied, jobs replaces any
jobspec found in command or arguments with the
corresponding process group ID, and executes command,
passing it arguments, returning its exit status.

Send a signal specified by sigspec or signum to the process
named by job specification jobspec or process IDpid.
sigspec is either a case-insensitive signal name such as
SIGINT (with or without the SIG prefix)
or a signal number; signum is a signal number.
If sigspec and signum are not present, SIGTERM is used.
The -l option lists the signal names.
If any arguments are supplied when -l is given, the names of the
signals corresponding to the arguments are listed, and the return status
is zero.
exit_status is a number specifying a signal number or the exit
status of a process terminated by a signal.
The -L option is equivalent to -l.
The return status is zero if at least one signal was successfully sent,
or non-zero if an error occurs or an invalid option is encountered.

wait

wait [-fn] [jobspec or pid …]

Wait until the child process specified by each process IDpid
or job specification jobspec exits and return the exit status of the
last command waited for.
If a job spec is given, all processes in the job are waited for.
If no arguments are given, all currently active child processes are
waited for, and the return status is zero.
If the -n option is supplied, wait waits for a single job
to terminate and returns its exit status.
Supplying the -f option, when job control is enabled,
forces wait to wait for each pid or jobspec to
terminate before returning its status, intead of returning when it changes
status.
If neither jobspec nor pid specifies an active child process
of the shell, the return status is 127.

disown

disown [-ar] [-h] [jobspec … | pid … ]

Without options, remove each jobspec from the table of
active jobs.
If the -h option is given, the job is not removed from the table,
but is marked so that SIGHUP is not sent to the job if the shell
receives a SIGHUP.
If jobspec is not present, and neither the -a nor the
-r option is supplied, the current job is used.
If no jobspec is supplied, the -a option means to remove or
mark all jobs; the -r option without a jobspec
argument restricts operation to running jobs.

suspend

suspend [-f]

Suspend the execution of this shell until it receives a
SIGCONT signal.
A login shell cannot be suspended; the -f
option can be used to override this and force the suspension.

When job control is not active, the kill and wait
builtins do not accept jobspec arguments. They must be
supplied process IDs.

7.3 Job Control Variables

auto_resume

This variable controls how the shell interacts with the user and
job control. If this variable exists then single word simple
commands without redirections are treated as candidates for resumption
of an existing job. There is no ambiguity allowed; if there is
more than one job beginning with the string typed, then
the most recently accessed job will be selected.
The name of a stopped job, in this context, is the command line
used to start it. If this variable is set to the value ‘exact’,
the string supplied must match the name of a stopped job exactly;
if set to ‘substring’,
the string supplied needs to match a substring of the name of a
stopped job. The ‘substring’ value provides functionality
analogous to the ‘%?’ job ID (see Job Control Basics).
If set to any other value, the supplied string must
be a prefix of a stopped job’s name; this provides functionality
analogous to the ‘%’ job ID.

8 Command Line Editing

This chapter describes the basic features of the GNU
command line editing interface.
Command line editing is provided by the Readline library, which is
used by several different programs, including Bash.
Command line editing is enabled by default when using an interactive shell,
unless the --noediting option is supplied at shell invocation.
Line editing is also used when using the -e option to the
read builtin command (see Bash Builtins).
By default, the line editing commands are similar to those of Emacs.
A vi-style line editing interface is also available.
Line editing can be enabled at any time using the -o emacs or
-o vi options to the set builtin command
(see The Set Builtin), or disabled using the +o emacs or
+o vi options to set.

8.1 Introduction to Line Editing

The following paragraphs describe the notation used to represent
keystrokes.

The text C-k is read as ‘Control-K’ and describes the character
produced when the k key is pressed while the Control key
is depressed.

The text M-k is read as ‘Meta-K’ and describes the character
produced when the Meta key (if you have one) is depressed, and the k
key is pressed.
The Meta key is labeled ALT on many keyboards.
On keyboards with two keys labeled ALT (usually to either side of
the space bar), the ALT on the left side is generally set to
work as a Meta key.
The ALT key on the right may also be configured to work as a
Meta key or may be configured as some other modifier, such as a
Compose key for typing accented characters.

If you do not have a Meta or ALT key, or another key working as
a Meta key, the identical keystroke can be generated by typing ESCfirst, and then typing k.
Either process is known as metafying the k key.

The text M-C-k is read as ‘Meta-Control-k’ and describes the
character produced by metafyingC-k.

In addition, several keys have their own names. Specifically,
DEL, ESC, LFD, SPC, RET, and TAB all
stand for themselves when seen in this text, or in an init file
(see Readline Init File).
If your keyboard lacks a LFD key, typing C-j will
produce the desired character.
The RET key may be labeled Return or Enter on
some keyboards.

8.2 Readline Interaction

Often during an interactive session you type in a long line of text,
only to notice that the first word on the line is misspelled. The
Readline library gives you a set of commands for manipulating the text
as you type it in, allowing you to just fix your typo, and not forcing
you to retype the majority of the line. Using these editing commands,
you move the cursor to the place that needs correction, and delete or
insert the text of the corrections. Then, when you are satisfied with
the line, you simply press RET. You do not have to be at the
end of the line to press RET; the entire line is accepted
regardless of the location of the cursor within the line.

8.2.1 Readline Bare Essentials

In order to enter characters into the line, simply type them. The typed
character appears where the cursor was, and then the cursor moves one
space to the right. If you mistype a character, you can use your
erase character to back up and delete the mistyped character.

Sometimes you may mistype a character, and
not notice the error until you have typed several other characters. In
that case, you can type C-b to move the cursor to the left, and then
correct your mistake. Afterwards, you can move the cursor to the right
with C-f.

When you add text in the middle of a line, you will notice that characters
to the right of the cursor are ‘pushed over’ to make room for the text
that you have inserted. Likewise, when you delete text behind the cursor,
characters to the right of the cursor are ‘pulled back’ to fill in the
blank space created by the removal of the text. A list of the bare
essentials for editing the text of an input line follows.

C-b

Move back one character.

C-f

Move forward one character.

DEL or Backspace

Delete the character to the left of the cursor.

C-d

Delete the character underneath the cursor.

Printing characters

Insert the character into the line at the cursor.

C-_ or C-x C-u

Undo the last editing command. You can undo all the way back to an
empty line.

(Depending on your configuration, the Backspace key be set to
delete the character to the left of the cursor and the DEL key set
to delete the character underneath the cursor, like C-d, rather
than the character to the left of the cursor.)

8.2.2 Readline Movement Commands

The above table describes the most basic keystrokes that you need
in order to do editing of the input line. For your convenience, many
other commands have been added in addition to C-b, C-f,
C-d, and DEL. Here are some commands for moving more rapidly
about the line.

C-a

Move to the start of the line.

C-e

Move to the end of the line.

M-f

Move forward a word, where a word is composed of letters and digits.

M-b

Move backward a word.

C-l

Clear the screen, reprinting the current line at the top.

Notice how C-f moves forward a character, while M-f moves
forward a word. It is a loose convention that control keystrokes
operate on characters while meta keystrokes operate on words.

8.2.3 Readline Killing Commands

Killing text means to delete the text from the line, but to save
it away for later use, usually by yanking (re-inserting)
it back into the line.
(‘Cut’ and ‘paste’ are more recent jargon for ‘kill’ and ‘yank’.)

If the description for a command says that it ‘kills’ text, then you can
be sure that you can get the text back in a different (or the same)
place later.

When you use a kill command, the text is saved in a kill-ring.
Any number of consecutive kills save all of the killed text together, so
that when you yank it back, you get it all. The kill
ring is not line specific; the text that you killed on a previously
typed line is available to be yanked back later, when you are typing
another line.

Here is the list of commands for killing text.

C-k

Kill the text from the current cursor position to the end of the line.

M-d

Kill from the cursor to the end of the current word, or, if between
words, to the end of the next word.
Word boundaries are the same as those used by M-f.

M-DEL

Kill from the cursor the start of the current word, or, if between
words, to the start of the previous word.
Word boundaries are the same as those used by M-b.

C-w

Kill from the cursor to the previous whitespace. This is different than
M-DEL because the word boundaries differ.

Here is how to yank the text back into the line. Yanking
means to copy the most-recently-killed text from the kill buffer.

C-y

Yank the most recently killed text back into the buffer at the cursor.

M-y

Rotate the kill-ring, and yank the new top. You can only do this if
the prior command is C-y or M-y.

8.2.4 Readline Arguments

You can pass numeric arguments to Readline commands. Sometimes the
argument acts as a repeat count, other times it is the sign of the
argument that is significant. If you pass a negative argument to a
command which normally acts in a forward direction, that command will
act in a backward direction. For example, to kill text back to the
start of the line, you might type ‘M-- C-k’.

The general way to pass numeric arguments to a command is to type meta
digits before the command. If the first ‘digit’ typed is a minus
sign (‘-’), then the sign of the argument will be negative. Once
you have typed one meta digit to get the argument started, you can type
the remainder of the digits, and then the command. For example, to give
the C-d command an argument of 10, you could type ‘M-1 0 C-d’,
which will delete the next ten characters on the input line.

8.2.5 Searching for Commands in the History

Readline provides commands for searching through the command history
(see Bash History Facilities)
for lines containing a specified string.
There are two search modes: incremental and non-incremental.

Incremental searches begin before the user has finished typing the
search string.
As each character of the search string is typed, Readline displays
the next entry from the history matching the string typed so far.
An incremental search requires only as many characters as needed to
find the desired history entry.
To search backward in the history for a particular string, type
C-r. Typing C-s searches forward through the history.
The characters present in the value of the isearch-terminators variable
are used to terminate an incremental search.
If that variable has not been assigned a value, the ESC and
C-J characters will terminate an incremental search.
C-g will abort an incremental search and restore the original line.
When the search is terminated, the history entry containing the
search string becomes the current line.

To find other matching entries in the history list, type C-r or
C-s as appropriate.
This will search backward or forward in the history for the next
entry matching the search string typed so far.
Any other key sequence bound to a Readline command will terminate
the search and execute that command.
For instance, a RET will terminate the search and accept
the line, thereby executing the command from the history list.
A movement command will terminate the search, make the last line found
the current line, and begin editing.

Readline remembers the last incremental search string. If two
C-rs are typed without any intervening characters defining a new
search string, any remembered search string is used.

Non-incremental searches read the entire search string before starting
to search for matching history lines. The search string may be
typed by the user or be part of the contents of the current line.

8.3 Readline Init File

Although the Readline library comes with a set of Emacs-like
keybindings installed by default, it is possible to use a different set
of keybindings.
Any user can customize programs that use Readline by putting
commands in an inputrc file, conventionally in his home directory.
The name of this
file is taken from the value of the shell variable INPUTRC. If
that variable is unset, the default is ~/.inputrc. If that
file does not exist or cannot be read, the ultimate default is
/etc/inputrc.
The bind builtin command can also be used to set Readline
keybindings and variables.
See Bash Builtins.

When a program which uses the Readline library starts up, the
init file is read, and the key bindings are set.

In addition, the C-x C-r command re-reads this init file, thus
incorporating any changes that you might have made to it.

8.3.1 Readline Init File Syntax

There are only a few basic constructs allowed in the
Readline init file. Blank lines are ignored.
Lines beginning with a ‘#’ are comments.
Lines beginning with a ‘$’ indicate conditional
constructs (see Conditional Init Constructs). Other lines
denote variable settings and key bindings.

Variable Settings

You can modify the run-time behavior of Readline by
altering the values of variables in Readline
using the set command within the init file.
The syntax is simple:

set variablevalue

Here, for example, is how to
change from the default Emacs-like key binding to use
vi line editing commands:

set editing-mode vi

Variable names and values, where appropriate, are recognized without regard
to case. Unrecognized variable names are ignored.

Boolean variables (those that can be set to on or off) are set to on if
the value is null or empty, on (case-insensitive), or 1. Any other
value results in the variable being set to off.

The bind -V command lists the current Readline variable names
and values. See Bash Builtins.

A great deal of run-time behavior is changeable with the following
variables.

bell-style

Controls what happens when Readline wants to ring the terminal bell.
If set to ‘none’, Readline never rings the bell. If set to
‘visible’, Readline uses a visible bell if one is available.
If set to ‘audible’ (the default), Readline attempts to ring
the terminal’s bell.

bind-tty-special-chars

If set to ‘on’ (the default), Readline attempts to bind the control
characters treated specially by the kernel’s terminal driver to their
Readline equivalents.

blink-matching-paren

If set to ‘on’, Readline attempts to briefly move the cursor to an
opening parenthesis when a closing parenthesis is inserted. The default
is ‘off’.

colored-completion-prefix

If set to ‘on’, when listing completions, Readline displays the
common prefix of the set of possible completions using a different color.
The color definitions are taken from the value of the LS_COLORS
environment variable.
The default is ‘off’.

colored-stats

If set to ‘on’, Readline displays possible completions using different
colors to indicate their file type.
The color definitions are taken from the value of the LS_COLORS
environment variable.
The default is ‘off’.

comment-begin

The string to insert at the beginning of the line when the
insert-comment command is executed. The default value
is "#".

completion-display-width

The number of screen columns used to display possible matches
when performing completion.
The value is ignored if it is less than 0 or greater than the terminal
screen width.
A value of 0 will cause matches to be displayed one per line.
The default value is -1.

completion-ignore-case

If set to ‘on’, Readline performs filename matching and completion
in a case-insensitive fashion.
The default value is ‘off’.

completion-map-case

If set to ‘on’, and completion-ignore-case is enabled, Readline
treats hyphens (‘-’) and underscores (‘_’) as equivalent when
performing case-insensitive filename matching and completion.
The default value is ‘off’.

completion-prefix-display-length

The length in characters of the common prefix of a list of possible
completions that is displayed without modification. When set to a
value greater than zero, common prefixes longer than this value are
replaced with an ellipsis when displaying possible completions.

completion-query-items

The number of possible completions that determines when the user is
asked whether the list of possibilities should be displayed.
If the number of possible completions is greater than this value,
Readline will ask the user whether or not he wishes to view
them; otherwise, they are simply listed.
This variable must be set to an integer value greater than or equal to 0.
A negative value means Readline should never ask.
The default limit is 100.

convert-meta

If set to ‘on’, Readline will convert characters with the
eighth bit set to an ASCII key sequence by stripping the eighth
bit and prefixing an ESC character, converting them to a
meta-prefixed key sequence. The default value is ‘on’, but
will be set to ‘off’ if the locale is one that contains
eight-bit characters.

disable-completion

If set to ‘On’, Readline will inhibit word completion.
Completion characters will be inserted into the line as if they had
been mapped to self-insert. The default is ‘off’.

echo-control-characters

When set to ‘on’, on operating systems that indicate they support it,
readline echoes a character corresponding to a signal generated from the
keyboard. The default is ‘on’.

editing-mode

The editing-mode variable controls which default set of
key bindings is used. By default, Readline starts up in Emacs editing
mode, where the keystrokes are most similar to Emacs. This variable can be
set to either ‘emacs’ or ‘vi’.

emacs-mode-string

If the show-mode-in-prompt variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when emacs editing mode is active. The value is expanded like a
key binding, so the standard set of meta- and control prefixes and
backslash escape sequences is available.
Use the ‘\1’ and ‘\2’ escapes to begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
The default is ‘@’.

enable-bracketed-paste

When set to ‘On’, Readline will configure the terminal in a way
that will enable it to insert each paste into the editing buffer as a
single string of characters, instead of treating each character as if
it had been read from the keyboard. This can prevent pasted characters
from being interpreted as editing commands. The default is ‘off’.

enable-keypad

When set to ‘on’, Readline will try to enable the application
keypad when it is called. Some systems need this to enable the
arrow keys. The default is ‘off’.

enable-meta-key

When set to ‘on’, Readline will try to enable any meta modifier
key the terminal claims to support when it is called. On many terminals,
the meta key is used to send eight-bit characters.
The default is ‘on’.

expand-tilde

If set to ‘on’, tilde expansion is performed when Readline
attempts word completion. The default is ‘off’.

history-preserve-point

If set to ‘on’, the history code attempts to place the point (the
current cursor position) at the
same location on each history line retrieved with previous-history
or next-history. The default is ‘off’.

history-size

Set the maximum number of history entries saved in the history list.
If set to zero, any existing history entries are deleted and no new entries
are saved.
If set to a value less than zero, the number of history entries is not
limited.
By default, the number of history entries is not limited.
If an attempt is made to set history-size to a non-numeric value,
the maximum number of history entries will be set to 500.

horizontal-scroll-mode

This variable can be set to either ‘on’ or ‘off’. Setting it
to ‘on’ means that the text of the lines being edited will scroll
horizontally on a single screen line when they are longer than the width
of the screen, instead of wrapping onto a new screen line. By default,
this variable is set to ‘off’.

input-meta

If set to ‘on’, Readline will enable eight-bit input (it
will not clear the eighth bit in the characters it reads),
regardless of what the terminal claims it can support. The
default value is ‘off’, but Readline will set it to ‘on’ if the
locale contains eight-bit characters.
The name meta-flag is a synonym for this variable.

isearch-terminators

The string of characters that should terminate an incremental search without
subsequently executing the character as a command (see Searching).
If this variable has not been given a value, the characters ESC and
C-J will terminate an incremental search.

keymap

Sets Readline’s idea of the current keymap for key binding commands.
Built-in keymap names are
emacs,
emacs-standard,
emacs-meta,
emacs-ctlx,
vi,
vi-move,
vi-command, and
vi-insert.
vi is equivalent to vi-command (vi-move is also a
synonym); emacs is equivalent to emacs-standard.
Applications may add additional names.
The default value is emacs.
The value of the editing-mode variable also affects the
default keymap.

keyseq-timeout

Specifies the duration Readline will wait for a character when reading an
ambiguous key sequence (one that can form a complete key sequence using
the input read so far, or can take additional input to complete a longer
key sequence).
If no input is received within the timeout, Readline will use the shorter
but complete key sequence.
Readline uses this value to determine whether or not input is
available on the current input source (rl_instream by default).
The value is specified in milliseconds, so a value of 1000 means that
Readline will wait one second for additional input.
If this variable is set to a value less than or equal to zero, or to a
non-numeric value, Readline will wait until another key is pressed to
decide which key sequence to complete.
The default value is 500.

mark-directories

If set to ‘on’, completed directory names have a slash
appended. The default is ‘on’.

mark-modified-lines

This variable, when set to ‘on’, causes Readline to display an
asterisk (‘*’) at the start of history lines which have been modified.
This variable is ‘off’ by default.

mark-symlinked-directories

If set to ‘on’, completed names which are symbolic links
to directories have a slash appended (subject to the value of
mark-directories).
The default is ‘off’.

match-hidden-files

This variable, when set to ‘on’, causes Readline to match files whose
names begin with a ‘.’ (hidden files) when performing filename
completion.
If set to ‘off’, the leading ‘.’ must be
supplied by the user in the filename to be completed.
This variable is ‘on’ by default.

menu-complete-display-prefix

If set to ‘on’, menu completion displays the common prefix of the
list of possible completions (which may be empty) before cycling through
the list. The default is ‘off’.

output-meta

If set to ‘on’, Readline will display characters with the
eighth bit set directly rather than as a meta-prefixed escape
sequence.
The default is ‘off’, but Readline will set it to ‘on’ if the
locale contains eight-bit characters.

page-completions

If set to ‘on’, Readline uses an internal more-like pager
to display a screenful of possible completions at a time.
This variable is ‘on’ by default.

print-completions-horizontally

If set to ‘on’, Readline will display completions with matches
sorted horizontally in alphabetical order, rather than down the screen.
The default is ‘off’.

revert-all-at-newline

If set to ‘on’, Readline will undo all changes to history lines
before returning when accept-line is executed. By default,
history lines may be modified and retain individual undo lists across
calls to readline. The default is ‘off’.

show-all-if-ambiguous

This alters the default behavior of the completion functions. If
set to ‘on’,
words which have more than one possible completion cause the
matches to be listed immediately instead of ringing the bell.
The default value is ‘off’.

show-all-if-unmodified

This alters the default behavior of the completion functions in
a fashion similar to show-all-if-ambiguous.
If set to ‘on’,
words which have more than one possible completion without any
possible partial completion (the possible completions don’t share
a common prefix) cause the matches to be listed immediately instead
of ringing the bell.
The default value is ‘off’.

show-mode-in-prompt

If set to ‘on’, add a string to the beginning of the prompt
indicating the editing mode: emacs, vi command, or vi insertion.
The mode strings are user-settable (e.g., emacs-mode-string).
The default value is ‘off’.

skip-completed-text

If set to ‘on’, this alters the default completion behavior when
inserting a single match into the line. It’s only active when
performing completion in the middle of a word. If enabled, readline
does not insert characters from the completion that match characters
after point in the word being completed, so portions of the word
following the cursor are not duplicated.
For instance, if this is enabled, attempting completion when the cursor
is after the ‘e’ in ‘Makefile’ will result in ‘Makefile’
rather than ‘Makefilefile’, assuming there is a single possible
completion.
The default value is ‘off’.

vi-cmd-mode-string

If the show-mode-in-prompt variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when vi editing mode is active and in command mode.
The value is expanded like a
key binding, so the standard set of meta- and control prefixes and
backslash escape sequences is available.
Use the ‘\1’ and ‘\2’ escapes to begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
The default is ‘(cmd)’.

vi-ins-mode-string

If the show-mode-in-prompt variable is enabled,
this string is displayed immediately before the last line of the primary
prompt when vi editing mode is active and in insertion mode.
The value is expanded like a
key binding, so the standard set of meta- and control prefixes and
backslash escape sequences is available.
Use the ‘\1’ and ‘\2’ escapes to begin and end sequences of
non-printing characters, which can be used to embed a terminal control
sequence into the mode string.
The default is ‘(ins)’.

visible-stats

If set to ‘on’, a character denoting a file’s type
is appended to the filename when listing possible
completions. The default is ‘off’.

Key Bindings

The syntax for controlling key bindings in the init file is
simple. First you need to find the name of the command that you
want to change. The following sections contain tables of the command
name, the default keybinding, if any, and a short description of what
the command does.

Once you know the name of the command, simply place on a line
in the init file the name of the key
you wish to bind the command to, a colon, and then the name of the
command.
There can be no space between the key name and the colon – that will be
interpreted as part of the key name.
The name of the key can be expressed in different ways, depending on
what you find most comfortable.

In addition to command names, readline allows keys to be bound
to a string that is inserted when the key is pressed (a macro).

The bind -p command displays Readline function names and
bindings in a format that can put directly into an initialization file.
See Bash Builtins.

In the example above, C-u is bound to the function
universal-argument,
M-DEL is bound to the function backward-kill-word, and
C-o is bound to run the macro
expressed on the right hand side (that is, to insert the text
‘> output’ into the line).

A number of symbolic character names are recognized while
processing this key binding syntax:
DEL,
ESC,
ESCAPE,
LFD,
NEWLINE,
RET,
RETURN,
RUBOUT,
SPACE,
SPC,
and
TAB.

"keyseq": function-name or macro

keyseq differs from keyname above in that strings
denoting an entire key sequence can be specified, by placing
the key sequence in double quotes. Some GNU Emacs style key
escapes can be used, as in the following example, but the
special character names are not recognized.

In the above example, C-u is again bound to the function
universal-argument (just as it was in the first example),
‘C-xC-r’ is bound to the function re-read-init-file,
and ‘ESC[11~’ is bound to insert
the text ‘Function Key 1’.

The following GNU Emacs style escape sequences are available when
specifying key sequences:

\C-

control prefix

\M-

meta prefix

\e

an escape character

\\

backslash

\"

", a double quotation mark

\'

', a single quote or apostrophe

In addition to the GNU Emacs style escape sequences, a second
set of backslash escapes is available:

\a

alert (bell)

\b

backspace

\d

delete

\f

form feed

\n

newline

\r

carriage return

\t

horizontal tab

\v

vertical tab

\nnn

the eight-bit character whose value is the octal value nnn
(one to three digits)

\xHH

the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)

When entering the text of a macro, single or double quotes must
be used to indicate a macro definition.
Unquoted text is assumed to be a function name.
In the macro body, the backslash escapes described above are expanded.
Backslash will quote any other character in the macro text,
including ‘"’ and ‘'’.
For example, the following binding will make ‘C-x \’
insert a single ‘\’ into the line:

8.3.2 Conditional Init Constructs

Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key
bindings and variable settings to be performed as the result
of tests. There are four parser directives used.

$if

The $if construct allows bindings to be made based on the
editing mode, the terminal being used, or the application using
Readline. The text of the test, after any comparison operator,
extends to the end of the line;
unless otherwise noted, no characters are required to isolate it.

mode

The mode= form of the $if directive is used to test
whether Readline is in emacs or vi mode.
This may be used in conjunction
with the ‘set keymap’ command, for instance, to set bindings in
the emacs-standard and emacs-ctlx keymaps only if
Readline is starting out in emacs mode.

term

The term= form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output by the
terminal’s function keys. The word on the right side of the
‘=’ is tested against both the full name of the terminal and
the portion of the terminal name before the first ‘-’. This
allows sun to match both sun and sun-cmd,
for instance.

version

The version test may be used to perform comparisons against
specific Readline versions.
The version expands to the current Readline version.
The set of comparison operators includes
‘=’ (and ‘==’), ‘!=’, ‘<=’, ‘>=’, ‘<’,
and ‘>’.
The version number supplied on the right side of the operator consists
of a major version number, an optional decimal point, and an optional
minor version (e.g., ‘7.1’). If the minor version is omitted, it
is assumed to be ‘0’.
The operator may be separated from the string version and
from the version number argument by whitespace.
The following example sets a variable if the Readline version being used
is 7.0 or newer:

$if version >= 7.0
set show-mode-in-prompt on
$endif

application

The application construct is used to include
application-specific settings. Each program using the Readline
library sets the application name, and you can test for
a particular value.
This could be used to bind key sequences to functions useful for
a specific program. For instance, the following command adds a
key sequence that quotes the current or previous word in Bash:

The variable construct provides simple equality tests for Readline
variables and values.
The permitted comparison operators are ‘=’, ‘==’, and ‘!=’.
The variable name must be separated from the comparison operator by
whitespace; the operator may be separated from the value on the right hand
side by whitespace.
Both string and boolean variables may be tested. Boolean variables must be
tested against the values on and off.
The following example is equivalent to the mode=emacs test described
above:

$if editing-mode == emacs
set show-mode-in-prompt on
$endif

$endif

This command, as seen in the previous example, terminates an
$if command.

$else

Commands in this branch of the $if directive are executed if
the test fails.

$include

This directive takes a single filename as an argument and reads commands
and bindings from that file.
For example, the following directive reads from /etc/inputrc:

This section describes Readline commands that may be bound to key
sequences.
You can list your key bindings by executing
bind -P or, for a more terse format, suitable for an
inputrc file, bind -p. (See Bash Builtins.)
Command names without an accompanying key sequence are unbound by default.

In the following descriptions, point refers to the current cursor
position, and mark refers to a cursor position saved by the
set-mark command.
The text between the point and mark is referred to as the region.

Move forward to the end of the next word.
Words are composed of letters and digits.

backward-word (M-b)

Move back to the start of the current or previous word.
Words are composed of letters and digits.

shell-forward-word ()

Move forward to the end of the next word.
Words are delimited by non-quoted shell metacharacters.

shell-backward-word ()

Move back to the start of the current or previous word.
Words are delimited by non-quoted shell metacharacters.

previous-screen-line ()

Attempt to move point to the same physical screen column on the previous
physical screen line. This will not have the desired effect if the current
Readline line does not take up more than one physical line or if point is not
greater than the length of the prompt plus the screen width.

next-screen-line ()

Attempt to move point to the same physical screen column on the next
physical screen line. This will not have the desired effect if the current
Readline line does not take up more than one physical line or if the length
of the current Readline line is not greater than the length of the prompt
plus the screen width.

clear-screen (C-l)

Clear the screen and redraw the current line,
leaving the current line at the top of the screen.

8.4.2 Commands For Manipulating The History

accept-line (Newline or Return)

Accept the line regardless of where the cursor is.
If this line is
non-empty, add it to the history list according to the setting of
the HISTCONTROL and HISTIGNORE variables.
If this line is a modified history line, then restore the history line
to its original state.

previous-history (C-p)

Move ‘back’ through the history list, fetching the previous command.

next-history (C-n)

Move ‘forward’ through the history list, fetching the next command.

beginning-of-history (M-<)

Move to the first line in the history.

end-of-history (M->)

Move to the end of the input history, i.e., the line currently
being entered.

reverse-search-history (C-r)

Search backward starting at the current line and moving ‘up’ through
the history as necessary. This is an incremental search.

forward-search-history (C-s)

Search forward starting at the current line and moving ‘down’ through
the history as necessary. This is an incremental search.

non-incremental-reverse-search-history (M-p)

Search backward starting at the current line and moving ‘up’
through the history as necessary using a non-incremental search
for a string supplied by the user.
The search string may match anywhere in a history line.

non-incremental-forward-search-history (M-n)

Search forward starting at the current line and moving ‘down’
through the history as necessary using a non-incremental search
for a string supplied by the user.
The search string may match anywhere in a history line.

history-search-forward ()

Search forward through the history for the string of characters
between the start of the current line and the point.
The search string must match at the beginning of a history line.
This is a non-incremental search.
By default, this command is unbound.

history-search-backward ()

Search backward through the history for the string of characters
between the start of the current line and the point.
The search string must match at the beginning of a history line.
This is a non-incremental search.
By default, this command is unbound.

history-substring-search-forward ()

Search forward through the history for the string of characters
between the start of the current line and the point.
The search string may match anywhere in a history line.
This is a non-incremental search.
By default, this command is unbound.

history-substring-search-backward ()

Search backward through the history for the string of characters
between the start of the current line and the point.
The search string may match anywhere in a history line.
This is a non-incremental search.
By default, this command is unbound.

yank-nth-arg (M-C-y)

Insert the first argument to the previous command (usually
the second word on the previous line) at point.
With an argument n,
insert the nth word from the previous command (the words
in the previous command begin with word 0). A negative argument
inserts the nth word from the end of the previous command.
Once the argument n is computed, the argument is extracted
as if the ‘!n’ history expansion had been specified.

yank-last-arg (M-. or M-_)

Insert last argument to the previous command (the last word of the
previous history entry).
With a numeric argument, behave exactly like yank-nth-arg.
Successive calls to yank-last-arg move back through the history
list, inserting the last word (or the word specified by the argument to
the first call) of each line in turn.
Any numeric argument supplied to these successive calls determines
the direction to move through the history. A negative argument switches
the direction through the history (back or forward).
The history expansion facilities are used to extract the last argument,
as if the ‘!$’ history expansion had been specified.

8.4.3 Commands For Changing Text

end-of-file (usually C-d)

The character indicating end-of-file as set, for example, by
stty. If this character is read when there are no characters
on the line, and point is at the beginning of the line, Readline
interprets it as the end of input and returns EOF.

delete-char (C-d)

Delete the character at point. If this function is bound to the
same character as the tty EOF character, as C-d
commonly is, see above for the effects.

backward-delete-char (Rubout)

Delete the character behind the cursor. A numeric argument means
to kill the characters instead of deleting them.

forward-backward-delete-char ()

Delete the character under the cursor, unless the cursor is at the
end of the line, in which case the character behind the cursor is
deleted. By default, this is not bound to a key.

quoted-insert (C-q or C-v)

Add the next character typed to the line verbatim. This is
how to insert key sequences like C-q, for example.

self-insert (a, b, A, 1, !, …)

Insert yourself.

bracketed-paste-begin ()

This function is intended to be bound to the "bracketed paste" escape
sequence sent by some terminals, and such a binding is assigned by default.
It allows Readline to insert the pasted text as a single unit without treating
each character as if it had been read from the keyboard. The characters
are inserted as if each one was bound to self-insert instead of
executing any editing commands.

transpose-chars (C-t)

Drag the character before the cursor forward over
the character at the cursor, moving the
cursor forward as well. If the insertion point
is at the end of the line, then this
transposes the last two characters of the line.
Negative arguments have no effect.

transpose-words (M-t)

Drag the word before point past the word after point,
moving point past that word as well.
If the insertion point is at the end of the line, this transposes
the last two words on the line.

upcase-word (M-u)

Uppercase the current (or following) word. With a negative argument,
uppercase the previous word, but do not move the cursor.

downcase-word (M-l)

Lowercase the current (or following) word. With a negative argument,
lowercase the previous word, but do not move the cursor.

capitalize-word (M-c)

Capitalize the current (or following) word. With a negative argument,
capitalize the previous word, but do not move the cursor.

In overwrite mode, characters bound to self-insert replace
the text at point rather than pushing the text to the right.
Characters bound to backward-delete-char replace the character
before point with a space.

8.4.5 Specifying Numeric Arguments

digit-argument (M-0, M-1, … M--)

Add this digit to the argument already accumulating, or start a new
argument. M-- starts a negative argument.

universal-argument ()

This is another way to specify an argument.
If this command is followed by one or more digits, optionally with a
leading minus sign, those digits define the argument.
If the command is followed by digits, executing universal-argument
again ends the numeric argument, but is otherwise ignored.
As a special case, if this command is immediately followed by a
character that is neither a digit nor minus sign, the argument count
for the next command is multiplied by four.
The argument count is initially one, so executing this function the
first time makes the argument count four, a second time makes the
argument count sixteen, and so on.
By default, this is not bound to a key.

8.4.6 Letting Readline Type For You

complete (TAB)

Attempt to perform completion on the text before point.
The actual completion performed is application-specific.
Bash attempts completion treating the text as a variable (if the
text begins with ‘$’), username (if the text begins with
‘~’), hostname (if the text begins with ‘@’), or
command (including aliases and functions) in turn. If none
of these produces a match, filename completion is attempted.

possible-completions (M-?)

List the possible completions of the text before point.
When displaying completions, Readline sets the number of columns used
for display to the value of completion-display-width, the value of
the environment variable COLUMNS, or the screen width, in that order.

insert-completions (M-*)

Insert all completions of the text before point that would have
been generated by possible-completions.

menu-complete ()

Similar to complete, but replaces the word to be completed
with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the list
of possible completions, inserting each match in turn.
At the end of the list of completions, the bell is rung
(subject to the setting of bell-style)
and the original text is restored.
An argument of n moves n positions forward in the list
of matches; a negative argument may be used to move backward
through the list.
This command is intended to be bound to TAB, but is unbound
by default.

menu-complete-backward ()

Identical to menu-complete, but moves backward through the list
of possible completions, as if menu-complete had been given a
negative argument.

delete-char-or-list ()

Deletes the character under the cursor if not at the beginning or
end of the line (like delete-char).
If at the end of the line, behaves identically to
possible-completions.
This command is unbound by default.

complete-filename (M-/)

Attempt filename completion on the text before point.

possible-filename-completions (C-x /)

List the possible completions of the text before point,
treating it as a filename.

complete-username (M-~)

Attempt completion on the text before point, treating
it as a username.

possible-username-completions (C-x ~)

List the possible completions of the text before point,
treating it as a username.

complete-variable (M-$)

Attempt completion on the text before point, treating
it as a shell variable.

possible-variable-completions (C-x $)

List the possible completions of the text before point,
treating it as a shell variable.

complete-hostname (M-@)

Attempt completion on the text before point, treating
it as a hostname.

possible-hostname-completions (C-x @)

List the possible completions of the text before point,
treating it as a hostname.

complete-command (M-!)

Attempt completion on the text before point, treating
it as a command name. Command completion attempts to
match the text against aliases, reserved words, shell
functions, shell builtins, and finally executable filenames,
in that order.

possible-command-completions (C-x !)

List the possible completions of the text before point,
treating it as a command name.

dynamic-complete-history (M-TAB)

Attempt completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.

dabbrev-expand ()

Attempt menu completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.

complete-into-braces (M-{)

Perform filename completion and insert the list of possible completions
enclosed within braces so the list is available to the shell
(see Brace Expansion).

8.4.8 Some Miscellaneous Commands

Read in the contents of the inputrc file, and incorporate
any bindings or variable assignments found there.

abort (C-g)

Abort the current editing command and
ring the terminal’s bell (subject to the setting of
bell-style).

do-lowercase-version (M-A, M-B, M-x, …)

If the metafied character x is upper case, run the command
that is bound to the corresponding metafied lower case character.
The behavior is undefined if x is already lower case.

prefix-meta (ESC)

Metafy the next character typed. This is for keyboards
without a meta key. Typing ‘ESC f’ is equivalent to typing
M-f.

undo (C-_ or C-x C-u)

Incremental undo, separately remembered for each line.

revert-line (M-r)

Undo all changes made to this line. This is like executing the undo
command enough times to get back to the beginning.

tilde-expand (M-&)

Perform tilde expansion on the current word.

set-mark (C-@)

Set the mark to the point. If a
numeric argument is supplied, the mark is set to that position.

exchange-point-and-mark (C-x C-x)

Swap the point with the mark. The current cursor position is set to
the saved position, and the old cursor position is saved as the mark.

character-search (C-])

A character is read and point is moved to the next occurrence of that
character. A negative count searches for previous occurrences.

character-search-backward (M-C-])

A character is read and point is moved to the previous occurrence
of that character. A negative count searches for subsequent
occurrences.

skip-csi-sequence ()

Read enough characters to consume a multi-key sequence such as those
defined for keys like Home and End. Such sequences begin with a
Control Sequence Indicator (CSI), usually ESC-[. If this sequence is
bound to "\e[", keys producing such sequences will have no effect
unless explicitly bound to a readline command, instead of inserting
stray characters into the editing buffer. This is unbound by default,
but usually bound to ESC-[.

insert-comment (M-#)

Without a numeric argument, the value of the comment-begin
variable is inserted at the beginning of the current line.
If a numeric argument is supplied, this command acts as a toggle: if
the characters at the beginning of the line do not match the value
of comment-begin, the value is inserted, otherwise
the characters in comment-begin are deleted from the beginning of
the line.
In either case, the line is accepted as if a newline had been typed.
The default value of comment-begin causes this command
to make the current line a shell comment.
If a numeric argument causes the comment character to be removed, the line
will be executed by the shell.

dump-functions ()

Print all of the functions and their key bindings to the
Readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file. This command is unbound by default.

dump-variables ()

Print all of the settable variables and their values to the
Readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file. This command is unbound by default.

dump-macros ()

Print all of the Readline key sequences bound to macros and the
strings they output. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file. This command is unbound by default.

glob-complete-word (M-g)

The word before point is treated as a pattern for pathname expansion,
with an asterisk implicitly appended. This pattern is used to
generate a list of matching file names for possible completions.

glob-expand-word (C-x *)

The word before point is treated as a pattern for pathname expansion,
and the list of matching file names is inserted, replacing the word.
If a numeric argument is supplied, a ‘*’ is appended before
pathname expansion.

glob-list-expansions (C-x g)

The list of expansions that would have been generated by
glob-expand-word is displayed, and the line is redrawn.
If a numeric argument is supplied, a ‘*’ is appended before
pathname expansion.

display-shell-version (C-x C-v)

Display version information about the current instance of Bash.

shell-expand-line (M-C-e)

Expand the line as the shell does.
This performs alias and history expansion as well as all of the shell
word expansions (see Shell Expansions).

Accept the current line for execution and fetch the next line
relative to the current line from the history for editing.
A numeric argument, if supplied, specifies the history entry to use instead
of the current line.

edit-and-execute-command (C-x C-e)

Invoke an editor on the current command line, and execute the result as shell
commands.
Bash attempts to invoke
$VISUAL, $EDITOR, and emacs
as the editor, in that order.

8.5 Readline vi Mode

While the Readline library does not have a full set of vi
editing functions, it does contain enough to allow simple editing
of the line. The Readline vi mode behaves as specified in
the POSIX standard.

In order to switch interactively between emacs and vi
editing modes, use the ‘set -o emacs’ and ‘set -o vi’
commands (see The Set Builtin).
The Readline default is emacs mode.

When you enter a line in vi mode, you are already placed in
‘insertion’ mode, as if you had typed an ‘i’. Pressing ESC
switches you into ‘command’ mode, where you can edit the text of the
line with the standard vi movement keys, move to previous
history lines with ‘k’ and subsequent lines with ‘j’, and
so forth.

8.6 Programmable Completion

When word completion is attempted for an argument to a command for
which a completion specification (a compspec) has been defined
using the complete builtin (see Programmable Completion Builtins),
the programmable completion facilities are invoked.

First, the command name is identified.
If a compspec has been defined for that command, the
compspec is used to generate the list of possible completions for the word.
If the command word is the empty string (completion attempted at the
beginning of an empty line), any compspec defined with
the -E option to complete is used.
If the command word is a full pathname, a compspec for the full
pathname is searched for first.
If no compspec is found for the full pathname, an attempt is made to
find a compspec for the portion following the final slash.
If those searches do not result in a compspec, any compspec defined with
the -D option to complete is used as the default.
If there is no default compspec, Bash attempts alias expansion
on the command word as a final resort, and attempts to find a compspec
for the command word from any successful expansion

Once a compspec has been found, it is used to generate the list of
matching words.
If a compspec is not found, the default Bash completion
described above (see Commands For Completion) is performed.

First, the actions specified by the compspec are used.
Only matches which are prefixed by the word being completed are
returned.
When the -f or -d option is used for filename or
directory name completion, the shell variable FIGNORE is
used to filter the matches.
See Bash Variables, for a description of FIGNORE.

Any completions specified by a filename expansion pattern to the
-G option are generated next.
The words generated by the pattern need not match the word being completed.
The GLOBIGNORE shell variable is not used to filter the matches,
but the FIGNORE shell variable is used.

Next, the string specified as the argument to the -W option
is considered.
The string is first split using the characters in the IFS
special variable as delimiters.
Shell quoting is honored within the string, in order to provide a
mechanism for the words to contain shell metacharacters or characters
in the value of IFS.
Each word is then expanded using
brace expansion, tilde expansion, parameter and variable expansion,
command substitution, and arithmetic expansion,
as described above (see Shell Expansions).
The results are split using the rules described above
(see Word Splitting).
The results of the expansion are prefix-matched against the word being
completed, and the matching words become the possible completions.

After these matches have been generated, any shell function or command
specified with the -F and -C options is invoked.
When the command or function is invoked, the COMP_LINE,
COMP_POINT, COMP_KEY, and COMP_TYPE variables are
assigned values as described above (see Bash Variables).
If a shell function is being invoked, the COMP_WORDS and
COMP_CWORD variables are also set.
When the function or command is invoked, the first argument ($1) is the
name of the command whose arguments are being completed, the
second argument ($2) is the word being completed, and the third argument
($3) is the word preceding the word being completed on the current command
line.
No filtering of the generated completions against the word being completed
is performed; the function or command has complete freedom in generating
the matches.

Any function specified with -F is invoked first.
The function may use any of the shell facilities, including the
compgen and compopt builtins described below
(see Programmable Completion Builtins), to generate the matches.
It must put the possible completions in the COMPREPLY array
variable, one per array element.

Next, any command specified with the -C option is invoked
in an environment equivalent to command substitution.
It should print a list of completions, one per line, to
the standard output.
Backslash may be used to escape a newline, if necessary.

After all of the possible completions are generated, any filter
specified with the -X option is applied to the list.
The filter is a pattern as used for pathname expansion; a ‘&’
in the pattern is replaced with the text of the word being completed.
A literal ‘&’ may be escaped with a backslash; the backslash
is removed before attempting a match.
Any completion that matches the pattern will be removed from the list.
A leading ‘!’ negates the pattern; in this case any completion
not matching the pattern will be removed.
If the nocasematch shell option
(see the description of shopt in The Shopt Builtin)
is enabled, the match is performed without regard to the case
of alphabetic characters.

Finally, any prefix and suffix specified with the -P and -S
options are added to each member of the completion list, and the result is
returned to the Readline completion code as the list of possible
completions.

If the previously-applied actions do not generate any matches, and the
-o dirnames option was supplied to complete when the
compspec was defined, directory name completion is attempted.

If the -o plusdirs option was supplied to complete when
the compspec was defined, directory name completion is attempted and any
matches are added to the results of the other actions.

By default, if a compspec is found, whatever it generates is returned to
the completion code as the full set of possible completions.
The default Bash completions are not attempted, and the Readline default
of filename completion is disabled.
If the -o bashdefault option was supplied to complete when
the compspec was defined, the default Bash completions are attempted
if the compspec generates no matches.
If the -o default option was supplied to complete when the
compspec was defined, Readline’s default completion will be performed
if the compspec (and, if attempted, the default Bash completions)
generate no matches.

When a compspec indicates that directory name completion is desired,
the programmable completion functions force Readline to append a slash
to completed names which are symbolic links to directories, subject to
the value of the mark-directories Readline variable, regardless
of the setting of the mark-symlinked-directories Readline variable.

There is some support for dynamically modifying completions. This is
most useful when used in combination with a default completion specified
with -D. It’s possible for shell functions executed as completion
handlers to indicate that completion should be retried by returning an
exit status of 124. If a shell function returns 124, and changes
the compspec associated with the command on which completion is being
attempted (supplied as the first argument when the function is executed),
programmable completion restarts from the beginning, with an
attempt to find a new compspec for that command. This allows a set of
completions to be built dynamically as completion is attempted, rather than
being loaded all at once.

For instance, assuming that there is a library of compspecs, each kept in a
file corresponding to the name of the command, the following default
completion function would load completions dynamically:

8.7 Programmable Completion Builtins

Three builtin commands are available to manipulate the programmable completion
facilities: one to specify how the arguments to a particular command are to
be completed, and two to modify the completion as it is happening.

compgen

compgen [option] [word]

Generate possible completion matches for word according to
the options, which may be any option accepted by the
complete
builtin with the exception of -p and -r, and write
the matches to the standard output.
When using the -F or -C options, the various shell variables
set by the programmable completion facilities, while available, will not
have useful values.

The matches will be generated in the same way as if the programmable
completion code had generated them directly from a completion specification
with the same flags.
If word is specified, only those completions matching word
will be displayed.

The return value is true unless an invalid option is supplied, or no
matches were generated.

Specify how arguments to each name should be completed.
If the -p option is supplied, or if no options are supplied, existing
completion specifications are printed in a way that allows them to be
reused as input.
The -r option removes a completion specification for
each name, or, if no names are supplied, all
completion specifications.
The -D option indicates that other supplied options and actions should
apply to the “default” command completion; that is, completion attempted
on a command for which no completion has previously been defined.
The -E option indicates that other supplied options and actions should
apply to “empty” command completion; that is, completion attempted on a
blank line.
The -I option indicates that other supplied options and actions should
apply to completion on the initial non-assignment word on the line, or after a
command delimiter such as ‘;’ or ‘|’, which is usually command
name completion.
If multiple options are supplied, the -D option takes precedence
over -E, and both take precedence over -I.
If any of -D, -E, or -I are supplied, any other
name arguments are ignored; these completions only apply to the case
specified by the option.

The process of applying these completion specifications when word completion
is attempted is described above (see Programmable Completion).

Other options, if specified, have the following meanings.
The arguments to the -G, -W, and -X options
(and, if necessary, the -P and -S options)
should be quoted to protect them from expansion before the
complete builtin is invoked.

-o comp-option

The comp-option controls several aspects of the compspec’s behavior
beyond the simple generation of completions.
comp-option may be one of:

bashdefault

Perform the rest of the default Bash completions if the compspec
generates no matches.

default

Use Readline’s default filename completion if the compspec generates
no matches.

dirnames

Perform directory name completion if the compspec generates no matches.

filenames

Tell Readline that the compspec generates filenames, so it can perform any
filename-specific processing (like adding a slash to directory names,
quoting special characters, or suppressing trailing spaces).
This option is intended to be used with shell functions specified
with -F.

noquote

Tell Readline not to quote the completed words if they are filenames
(quoting filenames is the default).

nosort

Tell Readline not to sort the list of possible completions alphabetically.

nospace

Tell Readline not to append a space (the default) to words completed at
the end of the line.

plusdirs

After any matches defined by the compspec are generated,
directory name completion is attempted and any
matches are added to the results of the other actions.

-A action

The action may be one of the following to generate a list of possible
completions:

command is executed in a subshell environment, and its output is
used as the possible completions.

-F function

The shell function function is executed in the current shell
environment.
When it is executed, $1 is the name of the command whose arguments are
being completed, $2 is the word being completed, and $3 is the word
preceding the word being completed, as described above
(see Programmable Completion).
When it finishes, the possible completions are retrieved from the value
of the COMPREPLY array variable.

-G globpat

The filename expansion pattern globpat is expanded to generate
the possible completions.

-P prefix

prefix is added at the beginning of each possible completion
after all other options have been applied.

-S suffix

suffix is appended to each possible completion
after all other options have been applied.

-W wordlist

The wordlist is split using the characters in the
IFS special variable as delimiters, and each resultant word
is expanded.
The possible completions are the members of the resultant list which
match the word being completed.

-X filterpat

filterpat is a pattern as used for filename expansion.
It is applied to the list of possible completions generated by the
preceding options and arguments, and each completion matching
filterpat is removed from the list.
A leading ‘!’ in filterpat negates the pattern; in this
case, any completion not matching filterpat is removed.

The return value is true unless an invalid option is supplied, an option
other than -p or -r is supplied without a name
argument, an attempt is made to remove a completion specification for
a name for which no specification exists, or
an error occurs adding a completion specification.

compopt

compopt [-o option] [-DEI] [+o option] [name]

Modify completion options for each name according to the
options, or for the currently-executing completion if no names
are supplied.
If no options are given, display the completion options for each
name or the current completion.
The possible values of option are those valid for the complete
builtin described above.
The -D option indicates that other supplied options should
apply to the “default” command completion; that is, completion attempted
on a command for which no completion has previously been defined.
The -E option indicates that other supplied options should
apply to “empty” command completion; that is, completion attempted on a
blank line.
The -I option indicates that other supplied options should
apply to completion on the initial non-assignment word on the line, or after a
command delimiter such as ‘;’ or ‘|’, which is usually command
name completion.

If multiple options are supplied, the -D option takes precedence
over -E, and both take precedence over -I

The return value is true unless an invalid option is supplied, an attempt
is made to modify the options for a name for which no completion
specification exists, or an output error occurs.

8.8 A Programmable Completion Example

The most common way to obtain additional completion functionality beyond
the default actions complete and compgen provide is to use
a shell function and bind it to a particular command using complete -F.

The following function provides completions for the cd builtin.
It is a reasonably good example of what shell functions must do when
used for completion. This function uses the word passed as $2
to determine the directory name to complete. You can also use the
COMP_WORDS array variable; the current word is indexed by the
COMP_CWORD variable.

The function relies on the complete and compgen builtins
to do much of the work, adding only the things that the Bash cd
does beyond accepting basic directory names:
tilde expansion (see Tilde Expansion),
searching directories in $CDPATH, which is described above
(see Bourne Shell Builtins),
and basic support for the cdable_vars shell option
(see The Shopt Builtin).
_comp_cd modifies the value of IFS so that it contains only
a newline to accommodate file names containing spaces and tabs –
compgen prints the possible completions it generates one per line.

Possible completions go into the COMPREPLY array variable, one
completion per array element. The programmable completion system retrieves
the completions from there when the function returns.

Since we’d like Bash and Readline to take care of some
of the other details for us, we use several other options to tell Bash
and Readline what to do. The -o filenames option tells Readline
that the possible completions should be treated as filenames, and quoted
appropriately. That option will also cause Readline to append a slash to
filenames it can determine are directories (which is why we might want to
extend _comp_cd to append a slash if we’re using directories found
via CDPATH: Readline can’t tell those completions are directories).
The -o nospace option tells Readline to not append a space
character to the directory name, in case we want to append to it.
The -o bashdefault option brings in the rest of the "Bash default"
completions – possible completion that Bash adds to the default Readline
set. These include things like command name completion, variable completion
for words beginning with ‘$’ or ‘${’, completions containing pathname
expansion patterns (see Filename Expansion), and so on.

Once installed using complete, _comp_cd will be called every
time we attempt word completion for a cd command.

Many more examples – an extensive collection of completions for most of
the common GNU, Unix, and Linux commands – are available as part of the
bash_completion project. This is installed by default on many GNU/Linux
distributions. Originally written by Ian Macdonald, the project now lives
at https://github.com/scop/bash-completion/. There are ports for
other systems such as Solaris and Mac OS X.

An older version of the bash_completion package is distributed with bash
in the examples/complete subdirectory.

9 Using History Interactively

This chapter describes how to use the GNU History Library
interactively, from a user’s standpoint.
It should be considered a user’s guide.
For information on using the GNU History Library in other programs,
see the GNU Readline Library Manual.

9.1 Bash History Facilities

When the -o history option to the set builtin
is enabled (see The Set Builtin),
the shell provides access to the command history,
the list of commands previously typed.
The value of the HISTSIZE shell variable is used as the
number of commands to save in a history list.
The text of the last $HISTSIZE
commands (default 500) is saved.
The shell stores each command in the history list prior to
parameter and variable expansion
but after history expansion is performed, subject to the
values of the shell variables
HISTIGNORE and HISTCONTROL.

When the shell starts up, the history is initialized from the
file named by the HISTFILE variable (default ~/.bash_history).
The file named by the value of HISTFILE is truncated, if
necessary, to contain no more than the number of lines specified by
the value of the HISTFILESIZE variable.
When a shell with history enabled exits, the last
$HISTSIZE lines are copied from the history list to the file
named by $HISTFILE.
If the histappend shell option is set (see Bash Builtins),
the lines are appended to the history file,
otherwise the history file is overwritten.
If HISTFILE
is unset, or if the history file is unwritable, the history is not saved.
After saving the history, the history file is truncated
to contain no more than $HISTFILESIZE lines.
If HISTFILESIZE is unset, or set to null, a non-numeric value, or
a numeric value less than zero, the history file is not truncated.

If the HISTTIMEFORMAT is set, the time stamp information
associated with each history entry is written to the history file,
marked with the history comment character.
When the history file is read, lines beginning with the history
comment character followed immediately by a digit are interpreted
as timestamps for the following history entry.

The builtin command fc may be used to list or edit and re-execute
a portion of the history list.
The history builtin may be used to display or modify the history
list and manipulate the history file.
When using command-line editing, search commands
are available in each editing mode that provide access to the
history list (see Commands For History).

The shell allows control over which commands are saved on the history
list. The HISTCONTROL and HISTIGNORE
variables may be set to cause the shell to save only a subset of the
commands entered.
The cmdhist
shell option, if enabled, causes the shell to attempt to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness.
The lithist
shell option causes the shell to save the command with embedded newlines
instead of semicolons.
The shopt builtin is used to set these options.
See The Shopt Builtin, for a description of shopt.

9.2 Bash History Builtins

Bash provides two builtin commands which manipulate the
history list and history file.

fc

fc [-e ename] [-lnr] [first] [last]fc -s [pat=rep] [command]

The first form selects a range of commands from first to
last from the history list and displays or edits and re-executes
them.
Both first and
last may be specified as a string (to locate the most recent
command beginning with that string) or as a number (an index into the
history list, where a negative number is used as an offset from the
current command number). If last is not specified, it is set to
first. If first is not specified, it is set to the previous
command for editing and -16 for listing. If the -l flag is
given, the commands are listed on standard output. The -n flag
suppresses the command numbers when listing. The -r flag
reverses the order of the listing. Otherwise, the editor given by
ename is invoked on a file containing those commands. If
ename is not given, the value of the following variable expansion
is used: ${FCEDIT:-${EDITOR:-vi}}. This says to use the
value of the FCEDIT variable if set, or the value of the
EDITOR variable if that is set, or vi if neither is set.
When editing is complete, the edited commands are echoed and executed.

In the second form, command is re-executed after each instance
of pat in the selected command is replaced by rep.
command is intepreted the same as first above.

A useful alias to use with the fc command is r='fc -s', so
that typing ‘r cc’ runs the last command beginning with cc
and typing ‘r’ re-executes the last command (see Aliases).

history

history [n]
history -c
history -d offset
history -d start-end
history [-anrw] [filename]
history -ps arg

With no options, display the history list with line numbers.
Lines prefixed with a ‘*’ have been modified.
An argument of n lists only the last n lines.
If the shell variable HISTTIMEFORMAT is set and not null,
it is used as a format string for strftime to display
the time stamp associated with each displayed history entry.
No intervening blank is printed between the formatted time stamp
and the history line.

Options, if supplied, have the following meanings:

-c

Clear the history list. This may be combined
with the other options to replace the history list completely.

-d offset

Delete the history entry at position offset.
If offset is positive, it should be specified as it appears when
the history is displayed.
If offset is negative, it is interpreted as relative to one greater
than the last history position, so negative indices count back from the
end of the history, and an index of ‘-1’ refers to the current
history -d command.

-d start-end

Delete the history entries between positions start and end,
inclusive. Positive and negative values for start and end
are interpreted as described above.

-a

Append the new history lines to the history file.
These are history lines entered since the beginning of the current
Bash session, but not already appended to the history file.

-n

Append the history lines not already read from the history file
to the current history list. These are lines appended to the history
file since the beginning of the current Bash session.

-r

Read the history file and append its contents to
the history list.

-w

Write out the current history list to the history file.

-p

Perform history substitution on the args and display the result
on the standard output, without storing the results in the history list.

-s

The args are added to the end of
the history list as a single entry.

When any of the -w, -r, -a, or -n options is
used, if filename
is given, then it is used as the history file. If not, then
the value of the HISTFILE variable is used.

9.3 History Expansion

The History library provides a history expansion feature that is similar
to the history expansion provided by csh. This section
describes the syntax used to manipulate the history information.

History expansions introduce words from the history list into
the input stream, making it easy to repeat commands, insert the
arguments to a previous command into the current input line, or
fix errors in previous commands quickly.

History expansion is performed immediately after a complete line
is read, before the shell breaks it into words, and is performed
on each line individually. Bash attempts to inform the history
expansion functions about quoting still in effect from previous lines.

History expansion takes place in two parts. The first is to determine
which line from the history list should be used during substitution.
The second is to select portions of that line for inclusion into the
current one. The line selected from the history is called the
event, and the portions of that line that are acted upon are
called words. Various modifiers are available to manipulate
the selected words. The line is broken into words in the same fashion
that Bash does, so that several words
surrounded by quotes are considered one word.
History expansions are introduced by the appearance of the
history expansion character, which is ‘!’ by default.

History expansion implements shell-like quoting conventions:
a backslash can be used to remove the special handling for the next character;
single quotes enclose verbatim sequences of characters, and can be used to
inhibit history expansion;
and characters enclosed within double quotes may be subject to history
expansion, since backslash can escape the history expansion character,
but single quotes may not, since they are not treated specially within
double quotes.

When using the shell, only ‘\’ and ‘'’ may be used to escape the
history expansion character, but the history expansion character is
also treated as quoted if it immediately precedes the closing double quote
in a double-quoted string.

Several shell options settable with the shopt
builtin (see The Shopt Builtin) may be used to tailor
the behavior of history expansion. If the
histverify shell option is enabled, and Readline
is being used, history substitutions are not immediately passed to
the shell parser.
Instead, the expanded line is reloaded into the Readline
editing buffer for further modification.
If Readline is being used, and the histreedit
shell option is enabled, a failed history expansion will be
reloaded into the Readline editing buffer for correction.
The -p option to the history builtin command
may be used to see what a history expansion will do before using it.
The -s option to the history builtin may be used to
add commands to the end of the history list without actually executing
them, so that they are available for subsequent recall.
This is most useful in conjunction with Readline.

The shell allows control of the various characters used by the
history expansion mechanism with the histchars variable,
as explained above (see Bash Variables). The shell uses
the history comment character to mark history timestamps when
writing the history file.

9.3.2 Word Designators

Word designators are used to select desired words from the event.
A ‘:’ separates the event specification from the word designator. It
may be omitted if the word designator begins with a ‘^’, ‘$’,
‘*’, ‘-’, or ‘%’. Words are numbered from the beginning
of the line, with the first word being denoted by 0 (zero). Words are
inserted into the current line separated by single spaces.

For example,

!!

designates the preceding command. When you type this, the preceding
command is repeated in toto.

!!:$

designates the last argument of the preceding command. This may be
shortened to !$.

!fi:2

designates the second argument of the most recent command starting with
the letters fi.

Here are the word designators:

0 (zero)

The 0th word. For many applications, this is the command word.

n

The nth word.

^

The first argument; that is, word 1.

$

The last argument.

%

The word matched by the most recent ‘?string?’ search.

x-y

A range of words; ‘-y’ abbreviates ‘0-y’.

*

All of the words, except the 0th. This is a synonym for ‘1-$’.
It is not an error to use ‘*’ if there is just one word in the event;
the empty string is returned in that case.

x*

Abbreviates ‘x-$’

x-

Abbreviates ‘x-$’ like ‘x*’, but omits the last word.

If a word designator is supplied without an event specification, the
previous command is used as the event.

9.3.3 Modifiers

After the optional word designator, you can add a sequence of one or more
of the following modifiers, each preceded by a ‘:’.

h

Remove a trailing pathname component, leaving only the head.

t

Remove all leading pathname components, leaving the tail.

r

Remove a trailing suffix of the form ‘.suffix’, leaving
the basename.

e

Remove all but the trailing suffix.

p

Print the new command but do not execute it.

q

Quote the substituted words, escaping further substitutions.

x

Quote the substituted words as with ‘q’,
but break into words at spaces, tabs, and newlines.

s/old/new/

Substitute new for the first occurrence of old in the
event line. Any delimiter may be used in place of ‘/’.
The delimiter may be quoted in old and new
with a single backslash. If ‘&’ appears in new,
it is replaced by old. A single backslash will quote
the ‘&’. The final delimiter is optional if it is the last
character on the input line.

&

Repeat the previous substitution.

g

a

Cause changes to be applied over the entire event line. Used in
conjunction with ‘s’, as in gs/old/new/,
or with ‘&’.

10 Installing Bash

This chapter provides basic instructions for installing Bash on
the various supported platforms. The distribution supports the
GNU operating systems, nearly every version of Unix, and several
non-Unix systems such as BeOS and Interix.
Other independent ports exist for
MS-DOS, OS/2, and Windows platforms.

10.1 Basic Installation

These are installation instructions for Bash.

The simplest way to compile Bash is:

cd to the directory containing the source code and type
‘./configure’ to configure Bash for your system. If you’re
using csh on an old version of System V, you might need to
type ‘sh ./configure’ instead to prevent csh from trying
to execute configure itself.

Running configure takes some time.
While running, it prints messages telling which features it is
checking for.

Type ‘make install’ to install bash and bashbug.
This will also install the manual pages and Info file.

The configure shell script attempts to guess correct
values for various system-dependent variables used during
compilation. It uses those values to create a Makefile in
each directory of the package (the top directory, the
builtins, doc, and support directories,
each directory under lib, and several others). It also creates a
config.h file containing system-dependent definitions.
Finally, it creates a shell script named config.status that you
can run in the future to recreate the current configuration, a
file config.cache that saves the results of its tests to
speed up reconfiguring, and a file config.log containing
compiler output (useful mainly for debugging configure).
If at some point
config.cache contains results you don’t want to keep, you
may remove or edit it.

To find out more about the options and arguments that the
configure script understands, type

bash-4.2$ ./configure --help

at the Bash prompt in your Bash source directory.

If you want to build Bash in a directory separate from the source
directory – to build for multiple architectures, for example –
just use the full path to the configure script. The following commands
will build bash in a directory under /usr/local/build from
the source code in /usr/local/src/bash-4.4:

If you need to do unusual things to compile Bash, please
try to figure out how configure could check whether or not
to do them, and mail diffs or instructions to
bash-maintainers@gnu.org so they can be
considered for the next release.

The file configure.ac is used to create configure
by a program called Autoconf. You only need
configure.ac if you want to change it or regenerate
configure using a newer version of Autoconf. If
you do this, make sure you are using Autoconf version 2.50 or
newer.

You can remove the program binaries and object files from the
source code directory by typing ‘make clean’. To also remove the
files that configure created (so you can compile Bash for
a different kind of computer), type ‘make distclean’.

10.2 Compilers and Options

Some systems require unusual options for compilation or linking
that the configure script does not know about. You can
give configure initial values for variables by setting
them in the environment. Using a Bourne-compatible shell, you
can do that on the command line like this:

10.3 Compiling For Multiple Architectures

You can compile Bash for more than one kind of computer at the
same time, by placing the object files for each architecture in their
own directory. To do this, you must use a version of make that
supports the VPATH variable, such as GNU make.
cd to the
directory where you want the object files and executables to go and run
the configure script from the source directory
(see Basic Installation).
You may need to
supply the --srcdir=PATH argument to tell configure where the
source files are. configure automatically checks for the
source code in the directory that configure is in and in ‘..’.

If you have to use a make that does not supports the VPATH
variable, you can compile Bash for one architecture at a
time in the source code directory. After you have installed
Bash for one architecture, use ‘make distclean’ before
reconfiguring for another architecture.

Alternatively, if your system supports symbolic links, you can use the
support/mkclone script to create a build tree which has
symbolic links back to each file in the source directory. Here’s an
example that creates a build directory in the current directory from a
source directory /usr/gnu/src/bash-2.0:

bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .

The mkclone script requires Bash, so you must have already built
Bash for at least one architecture before you can create build
directories for other architectures.

10.4 Installation Names

By default, ‘make install’ will install into
/usr/local/bin, /usr/local/man, etc. You can
specify an installation prefix other than /usr/local by
giving configure the option --prefix=PATH,
or by specifying a value for the DESTDIR ‘make’
variable when running ‘make install’.

You can specify separate installation prefixes for
architecture-specific files and architecture-independent files.
If you give configure the option
--exec-prefix=PATH, ‘make install’ will use
PATH as the prefix for installing programs and libraries.
Documentation and other data files will still use the regular prefix.

10.5 Specifying the System Type

There may be some features configure can not figure out
automatically, but need to determine by the type of host Bash
will run on. Usually configure can figure that
out, but if it prints a message saying it can not guess the host
type, give it the --host=TYPE option. ‘TYPE’ can
either be a short name for the system type, such as ‘sun4’,
or a canonical name with three fields: ‘CPU-COMPANY-SYSTEM’
(e.g., ‘i386-unknown-freebsd4.2’).

See the file support/config.sub for the possible
values of each field.

10.6 Sharing Defaults

If you want to set default values for configure scripts to
share, you can create a site shell script called
config.site that gives default values for variables like
CC, cache_file, and prefix. configure
looks for PREFIX/share/config.site if it exists, then
PREFIX/etc/config.site if it exists. Or, you can set the
CONFIG_SITE environment variable to the location of the site
script. A warning: the Bash configure looks for a site script,
but not all configure scripts do.

10.8 Optional Features

The Bash configure has a number of --enable-feature
options, where feature indicates an optional part of Bash.
There are also several --with-package options,
where package is something like ‘bash-malloc’ or ‘purify’.
To turn off the default use of a package, use
--without-package. To configure Bash without a feature
that is enabled by default, use --disable-feature.

Here is a complete list of the --enable- and
--with- options that the Bash configure recognizes.

--with-afs

Define if you are using the Andrew File System from Transarc.

--with-bash-malloc

Use the Bash version of
malloc in the directory lib/malloc. This is not the same
malloc that appears in GNU libc, but an older version
originally derived from the 4.2 BSDmalloc. This malloc
is very fast, but wastes some space on each allocation.
This option is enabled by default.
The NOTES file contains a list of systems for
which this should be turned off, and configure disables this
option automatically for a number of systems.

--with-curses

Use the curses library instead of the termcap library. This should
be supplied if your system has an inadequate or incomplete termcap
database.

--with-gnu-malloc

A synonym for --with-bash-malloc.

--with-installed-readline[=PREFIX]

Define this to make Bash link with a locally-installed version of Readline
rather than the version in lib/readline. This works only with
Readline 5.0 and later versions. If PREFIX is yes or not
supplied, configure uses the values of the make variables
includedir and libdir, which are subdirectories of prefix
by default, to find the installed version of Readline if it is not in
the standard system include and library directories.
If PREFIX is no, Bash links with the version in
lib/readline.
If PREFIX is set to any other value, configure treats it as
a directory pathname and looks for
the installed version of Readline in subdirectories of that directory
(include files in PREFIX/include and the library in
PREFIX/lib).

--with-purify

Define this to use the Purify memory allocation checker from Rational
Software.

--enable-minimal-config

This produces a shell with minimal features, close to the historical
Bourne shell.

There are several --enable- options that alter how Bash is
compiled and linked, rather than changing run-time features.

--enable-largefile

Enable support for large files if the operating system requires special compiler options
to build programs which can access large files. This is enabled by
default, if the operating system provides large file support.

--enable-profiling

This builds a Bash binary that produces profiling information to be
processed by gprof each time it is executed.

--enable-static-link

This causes Bash to be linked statically, if gcc is being used.
This could be used to build a version to use as root’s shell.

The ‘minimal-config’ option can be used to disable all of
the following options, but it is processed first, so individual
options may be enabled using ‘enable-feature’.

All of the following options except for ‘disabled-builtins’,
‘direxpand-default’, and
‘xpg-echo-default’ are
enabled by default, unless the operating system does not provide the
necessary support.

--enable-alias

Allow alias expansion and include the alias and unalias
builtins (see Aliases).

--enable-arith-for-command

Include support for the alternate form of the for command
that behaves like the C language for statement
(see Looping Constructs).

--enable-array-variables

Include support for one-dimensional array shell variables
(see Arrays).

Include support for case-modifying attributes in the declare builtin
and assignment statements. Variables with the uppercase attribute,
for example, will have their values converted to uppercase upon assignment.

--enable-casemod-expansion

Include support for case-modifying word expansions.

--enable-command-timing

Include support for recognizing time as a reserved word and for
displaying timing statistics for the pipeline following time
(see Pipelines).
This allows pipelines as well as shell builtins and functions to be timed.

Include support for matching POSIX regular expressions using the
‘=~’ binary operator in the [[ conditional command.
(see Conditional Constructs).

--enable-coprocesses

Include support for coprocesses and the coproc reserved word
(see Pipelines).

--enable-debugger

Include support for the bash debugger (distributed separately).

--enable-dev-fd-stat-broken

If calling stat on /dev/fd/N returns different results than
calling fstat on file descriptor N, supply this option to
enable a workaround.
This has implications for conditional commands that test file attributes.

--enable-direxpand-default

Cause the direxpand shell option (see The Shopt Builtin)
to be enabled by default when the shell starts.
It is normally disabled by default.

--enable-directory-stack

Include support for a csh-like directory stack and the
pushd, popd, and dirs builtins
(see The Directory Stack).

--enable-disabled-builtins

Allow builtin commands to be invoked via ‘builtin xxx’
even after xxx has been disabled using ‘enable -n xxx’.
See Bash Builtins, for details of the builtin and
enable builtin commands.

Include support for the extended pattern matching features described
above under Pattern Matching.

--enable-extended-glob-default

Set the default value of the extglob shell option described
above under The Shopt Builtin to be enabled.

--enable-function-import

Include support for importing function definitions exported by another
instance of the shell from the environment. This option is enabled by
default.

--enable-glob-asciirange-default

Set the default value of the globasciiranges shell option described
above under The Shopt Builtin to be enabled.
This controls the behavior of character ranges when used in pattern matching
bracket expressions.

--enable-help-builtin

Include the help builtin, which displays help on shell builtins and
variables (see Bash Builtins).

This enables the job control features (see Job Control),
if the operating system supports them.

--enable-multibyte

This enables support for multibyte characters if the operating
system provides the necessary support.

--enable-net-redirections

This enables the special handling of filenames of the form
/dev/tcp/host/port and
/dev/udp/host/port
when used in redirections (see Redirections).

--enable-process-substitution

This enables process substitution (see Process Substitution) if
the operating system provides the necessary support.

--enable-progcomp

Enable the programmable completion facilities
(see Programmable Completion).
If Readline is not enabled, this option has no effect.

--enable-prompt-string-decoding

Turn on the interpretation of a number of backslash-escaped characters
in the $PS0, $PS1, $PS2, and $PS4 prompt
strings. See Controlling the Prompt, for a complete list of prompt
string escape sequences.

--enable-readline

Include support for command-line editing and history with the Bash
version of the Readline library (see Command Line Editing).

--enable-restricted

Include support for a restricted shell. If this is enabled, Bash,
when called as rbash, enters a restricted mode. See
The Restricted Shell, for a description of restricted mode.

--enable-select

Include the select compound command, which allows the generation of
simple menus (see Conditional Constructs).

--enable-separate-helpfiles

Use external files for the documentation displayed by the help builtin
instead of storing the text internally.

--enable-single-help-strings

Store the text displayed by the help builtin as a single string for
each help topic. This aids in translating the text to different languages.
You may need to disable this if your compiler cannot handle very long string
literals.

Make the echo builtin expand backslash-escaped characters by default,
without requiring the -e option.
This sets the default value of the xpg_echo shell option to on,
which makes the Bash echo behave more like the version specified in
the Single Unix Specification, version 3.
See Bash Builtins, for a description of the escape sequences that
echo recognizes.

The file config-top.h contains C Preprocessor
‘#define’ statements for options which are not settable from
configure.
Some of these are not meant to be changed; beware of the consequences if
you do.
Read the comments associated with each definition for more
information about its effect.

Appendix A Reporting Bugs

Please report all bugs you find in Bash.
But first, you should
make sure that it really is a bug, and that it appears in the latest
version of Bash.
The latest version of Bash is always available for FTP from
ftp://ftp.gnu.org/pub/gnu/bash/.

Once you have determined that a bug actually exists, use the
bashbug command to submit a bug report.
If you have a fix, you are encouraged to mail that as well!
Suggestions and ‘philosophical’ bug reports may be mailed
to bug-bash@gnu.org or posted to the Usenet
newsgroup gnu.bash.bug.

All bug reports should include:

The version number of Bash.

The hardware and operating system.

The compiler used to compile Bash.

A description of the bug behaviour.

A short script or ‘recipe’ which exercises the bug and may be used
to reproduce it.

bashbug inserts the first three items automatically into
the template it provides for filing a bug report.

Appendix B Major Differences From The Bourne Shell

Bash implements essentially the same grammar, parameter and
variable expansion, redirection, and quoting as the Bourne Shell.
Bash uses the POSIX standard as the specification of
how these features are to be implemented. There are some
differences between the traditional Bourne shell and Bash; this
section quickly details the differences of significance. A
number of these differences are explained in greater depth in
previous sections.
This section uses the version of sh included in SVR4.2 (the
last version of the historical Bourne shell) as the baseline reference.

Bash has command history (see Bash History Facilities) and the
history and fc builtins to manipulate it.
The Bash history list maintains timestamp information and uses the
value of the HISTTIMEFORMAT variable to display it.

Bash has one-dimensional array variables (see Arrays), and the
appropriate variable expansions and assignment syntax to use them.
Several of the Bash builtins take options to act on arrays.
Bash provides a number of built-in array variables.

The $'…' quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes,
is supported (see ANSI-C Quoting).

Bash supports the $"…" quoting syntax to do
locale-specific translation of the characters between the double
quotes. The -D, --dump-strings, and --dump-po-strings
invocation options list the translatable strings found in a script
(see Locale Translation).

Bash implements the ! keyword to negate the return value of
a pipeline (see Pipelines).
Very useful when an if statement needs to act only if a test fails.
The Bash ‘-o pipefail’ option to set will cause a pipeline to
return a failure status if any command fails.

Bash has the time reserved word and command timing (see Pipelines).
The display of the timing statistics may be controlled with the
TIMEFORMAT variable.

Variables present in the shell’s initial environment are automatically
exported to child processes. The Bourne shell does not normally do
this unless the variables are explicitly marked using the export
command.

Bash supports the ‘+=’ assignment operator, which appends to the value
of the variable named on the left hand side.

Bash automatically assigns variables that provide information about the
current user (UID, EUID, and GROUPS), the current host
(HOSTTYPE, OSTYPE, MACHTYPE, and HOSTNAME),
and the instance of Bash that is running (BASH,
BASH_VERSION, and BASH_VERSINFO). See Bash Variables,
for details.

The IFS variable is used to split only the results of expansion,
not all words (see Word Splitting).
This closes a longstanding shell security hole.

The filename expansion bracket expression code uses ‘!’ and ‘^’
to negate the set of characters between the brackets.
The Bourne shell uses only ‘!’.

Bash implements the full set of POSIX filename expansion operators,
including character classes, equivalence classes, and
collating symbols (see Filename Expansion).

It is possible to have a variable and a function with the same name;
sh does not separate the two name spaces.

Bash functions are permitted to have local variables using the
local builtin, and thus useful recursive functions may be written
(see Bash Builtins).

Variable assignments preceding commands affect only that command, even
builtins and functions (see Environment).
In sh, all variable assignments
preceding commands are global unless the command is executed from the
file system.

Bash contains the ‘<>’ redirection operator, allowing a file to be
opened for both reading and writing, and the ‘&>’ redirection
operator, for directing standard output and standard error to the same
file (see Redirections).

Bash includes the ‘<<<’ redirection operator, allowing a string to
be used as the standard input to a command.

Bash implements the ‘[n]<&word’ and ‘[n]>&word’
redirection operators, which move one file descriptor to another.

Bash treats a number of filenames specially when they are
used in redirection operators (see Redirections).

Bash can open network connections to arbitrary machines and services
with the redirection operators (see Redirections).

The noclobber option is available to avoid overwriting existing
files with output redirection (see The Set Builtin).
The ‘>|’ redirection operator may be used to override noclobber.

The Bash cd and pwd builtins (see Bourne Shell Builtins)
each take -L and -P options to switch between logical and
physical modes.

Bash allows a function to override a builtin with the same name, and provides
access to that builtin’s functionality within the function via the
builtin and command builtins (see Bash Builtins).

The command builtin allows selective disabling of functions
when command lookup is performed (see Bash Builtins).

Individual builtins may be enabled or disabled using the enable
builtin (see Bash Builtins).

The Bash exec builtin takes additional options that allow users
to control the contents of the environment passed to the executed
command, and what the zeroth argument to the command is to be
(see Bourne Shell Builtins).

Shell functions may be exported to children via the environment
using export -f (see Shell Functions).

The Bash export, readonly, and declare builtins can
take a -f option to act on shell functions, a -p option to
display variables with various attributes set in a format that can be
used as shell input, a -n option to remove various variable
attributes, and ‘name=value’ arguments to set variable attributes
and values simultaneously.

The Bash hash builtin allows a name to be associated with
an arbitrary filename, even when that filename cannot be found by
searching the $PATH, using ‘hash -p’
(see Bourne Shell Builtins).

Bash includes a help builtin for quick reference to shell
facilities (see Bash Builtins).

The printf builtin is available to display formatted output
(see Bash Builtins).

The Bash read builtin (see Bash Builtins)
will read a line ending in ‘\’ with
the -r option, and will use the REPLY variable as a
default if no non-option arguments are supplied.
The Bash read builtin
also accepts a prompt string with the -p option and will use
Readline to obtain the line when given the -e option.
The read builtin also has additional options to control input:
the -s option will turn off echoing of input characters as
they are read, the -t option will allow read to time out
if input does not arrive within a specified number of seconds, the
-n option will allow reading only a specified number of
characters rather than a full line, and the -d option will read
until a particular character rather than newline.

The return builtin may be used to abort execution of scripts
executed with the . or source builtins
(see Bourne Shell Builtins).

Bash includes the shopt builtin, for finer control of shell
optional capabilities (see The Shopt Builtin), and allows these options
to be set and unset at shell invocation (see Invoking Bash).

Bash has much more optional behavior controllable with the set
builtin (see The Set Builtin).

The ‘-x’ (xtrace) option displays commands other than
simple commands when performing an execution trace
(see The Set Builtin).

The test builtin (see Bourne Shell Builtins)
is slightly different, as it implements the POSIX algorithm,
which specifies the behavior based on the number of arguments.

Bash includes the caller builtin, which displays the context of
any active subroutine call (a shell function or a script executed with
the . or source builtins). This supports the bash
debugger.

The trap builtin (see Bourne Shell Builtins) allows a
DEBUG pseudo-signal specification, similar to EXIT.
Commands specified with a DEBUG trap are executed before every
simple command, for command, case command,
select command, every arithmetic for command, and before
the first command executes in a shell function.
The DEBUG trap is not inherited by shell functions unless the
function has been given the trace attribute or the
functrace option has been enabled using the shopt builtin.
The extdebug shell option has additional effects on the
DEBUG trap.

The trap builtin (see Bourne Shell Builtins) allows an
ERR pseudo-signal specification, similar to EXIT and DEBUG.
Commands specified with an ERR trap are executed after a simple
command fails, with a few exceptions.
The ERR trap is not inherited by shell functions unless the
-o errtrace option to the set builtin is enabled.

The trap builtin (see Bourne Shell Builtins) allows a
RETURN pseudo-signal specification, similar to
EXIT and DEBUG.
Commands specified with an RETURN trap are executed before
execution resumes after a shell function or a shell script executed with
. or source returns.
The RETURN trap is not inherited by shell functions unless the
function has been given the trace attribute or the
functrace option has been enabled using the shopt builtin.

The Bash type builtin is more extensive and gives more information
about the names it finds (see Bash Builtins).

The Bash umask builtin permits a -p option to cause
the output to be displayed in the form of a umask command
that may be reused as input (see Bourne Shell Builtins).

Bash implements a csh-like directory stack, and provides the
pushd, popd, and dirs builtins to manipulate it
(see The Directory Stack).
Bash also makes the directory stack visible as the value of the
DIRSTACK shell variable.

Bash interprets special backslash-escaped characters in the prompt
strings when interactive (see Controlling the Prompt).

The Bash restricted mode is more useful (see The Restricted Shell);
the SVR4.2 shell restricted mode is too limited.

The disown builtin can remove a job from the internal shell
job table (see Job Control Builtins) or suppress the sending
of SIGHUP to a job when the shell exits as the result of a
SIGHUP.

Bash includes a number of features to support a separate debugger for
shell scripts.

The SVR4.2 shell has two privilege-related builtins
(mldmode and priv) not present in Bash.

B.1 Implementation Differences From The SVR4.2 Shell

Since Bash is a completely new implementation, it does not suffer from
many of the limitations of the SVR4.2 shell. For instance:

Bash does not fork a subshell when redirecting into or out of
a shell control structure such as an if or while
statement.

Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
insert a needed closing quote at EOF under certain circumstances.
This can be the cause of some hard-to-find errors.

The SVR4.2 shell uses a baroque memory management scheme based on
trapping SIGSEGV. If the shell is started from a process with
SIGSEGV blocked (e.g., by using the system() C library
function call), it misbehaves badly.

In a questionable attempt at security, the SVR4.2 shell,
when invoked without the -p option, will alter its real
and effective UID and GID if they are less than some
magic threshold value, commonly 100.
This can lead to unexpected results.

The SVR4.2 shell does not allow users to trap SIGSEGV,
SIGALRM, or SIGCHLD.

The SVR4.2 shell does not allow the IFS, MAILCHECK,
PATH, PS1, or PS2 variables to be unset.

The SVR4.2 shell treats ‘^’ as the undocumented equivalent of
‘|’.

Bash allows multiple option arguments when it is invoked (-x -v);
the SVR4.2 shell allows only one option argument (-xv). In
fact, some versions of the shell dump core if the second argument begins
with a ‘-’.

The SVR4.2 shell exits a script if any builtin fails; Bash exits
a script only if one of the POSIX special builtins fails, and
only for certain failures, as enumerated in the POSIX standard.

The purpose of this License is to make a manual, textbook, or other
functional and useful document free in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.

This License is a kind of “copyleft”, which means that derivative
works of the document must themselves be free in the same sense. It
complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does. But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License
principally for works whose purpose is instruction or reference.

APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License. Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein. The “Document”, below,
refers to any such manual or work. Any member of the public is a
licensee, and is addressed as “you”. You accept the license if you
copy, modify or distribute the work in a way requiring permission
under copyright law.

A “Modified Version” of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.

A “Secondary Section” is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document’s overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject. (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.) The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.

The “Invariant Sections” are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License. If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant. The Document may contain zero
Invariant Sections. If the Document does not identify any Invariant
Sections then there are none.

The “Cover Texts” are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License. A Front-Cover Text may
be at most 5 words, and a Back-Cover Text may be at most 25 words.

A “Transparent” copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters. A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text. A copy that is not “Transparent” is called “Opaque”.

Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input
format, SGML or XML using a publicly available
DTD, and standard-conforming simple HTML,
PostScript or PDF designed for human modification. Examples
of transparent image formats include PNG, XCF and
JPG. Opaque formats include proprietary formats that can be
read and edited only by proprietary word processors, SGML or
XML for which the DTD and/or processing tools are
not generally available, and the machine-generated HTML,
PostScript or PDF produced by some word processors for
output purposes only.

The “Title Page” means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page. For works in
formats which do not have any title page as such, “Title Page” means
the text near the most prominent appearance of the work’s title,
preceding the beginning of the body of the text.

The “publisher” means any person or entity that distributes copies
of the Document to the public.

A section “Entitled XYZ” means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language. (Here XYZ stands for a
specific section name mentioned below, such as “Acknowledgements”,
“Dedications”, “Endorsements”, or “History”.) To “Preserve the Title”
of such a section when you modify the Document means that it remains a
section “Entitled XYZ” according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document’s license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify
you as the publisher of these copies. The front cover must present
the full title with all words of the title equally prominent and
visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:

Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document). You may use the same title as a previous version
if the original publisher of that version gives permission.

List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

State on the Title page the name of the publisher of the
Modified Version, as the publisher.

Preserve all the copyright notices of the Document.

Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document’s license notice.

Include an unaltered copy of this License.

Preserve the section Entitled “History”, Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page. If
there is no section Entitled “History” in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on. These may be placed in the “History” section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

For any section Entitled “Acknowledgements” or “Dedications”, Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers
or the equivalent are not considered part of the section titles.

Delete any section Entitled “Endorsements”. Such a section
may not be included in the Modified Version.

Do not retitle any existing section to be Entitled “Endorsements” or
to conflict in title with any Invariant Section.

Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version’s license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled “Endorsements”, provided it contains
nothing but endorsements of your Modified Version by various
parties—for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled “History”
in the various original documents, forming one section Entitled
“History”; likewise combine any sections Entitled “Acknowledgements”,
and any sections Entitled “Dedications”. You must delete all
sections Entitled “Endorsements.”

COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an “aggregate” if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation’s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document’s Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled “Acknowledgements”,
“Dedications”, or “History”, the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense, or distribute it is void, and
will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.

Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.

FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License “or any later version” applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation. If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy’s public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.

RELICENSING

“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server. A
“Massive Multiauthor Collaboration” (or “MMC”) contained in the
site means any set of copyrightable works thus published on the MMC
site.

“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

“Incorporate” means to publish or republish a Document, in whole or
in part, as part of another Document.

An MMC is “eligible for relicensing” if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

Copyright (C) yearyour name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the “with…Texts.” line with this:

with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.